Most of us are well aware that chocolate is already a bit addictive to human beings... maybe it's the caffeine or the sugar? But what if there were a species to which chocolate was as addictive as heroin? Not only is it the best smelling/tasting and enticing thing they've ever encountered, but the first time they try it, they are instantly hooked. Attempting to forgo chocolate results in severe withdrawal symptoms. However, other human sweets/foods do not cause this, unless there is chocolate in them. Only chocolate, specifically. Other than the cravings, withdrawals, and a surge of "happiness" upon consuming chocolate (endorphins, probably), chocolate doesn't have any other typically associated drug-like effects such as hallucinations or motor function impairment.

What could be causing this phenomenon? Assuming the species functions under the same basic physiological systems we generally understand in life as we know it, as opposed to being a weird fifth-dimensional crystalline-based goo being or some other fantasy dodge. Is there any way to explain or justify this extreme addictive effect?

Theobromine

Theobromine...is a bitter alkaloid of the cacao plant...It is
classified as a xanthine alkaloid, others of which include
theophylline and caffeine. The compounds differ in that caffeine has
an extra methyl group. (ref)

While smaller amounts of theobromine can be found in other foods (like tea) and caffeine can break down into theobromine, the largest source is from chocolate.

Theobromine is not considered addictive in humans, but it certainly does have some effects.

The contributions of theobromine are less clear and its psychoactive
effects appear subtle...Although two early
studies failed to detect psychopharmacological activity...[one] found that 5 of 7
participants were able to discriminate 560 mg theobromine from placebo
or caffeine, suggesting that theobromine might be about one tenth as
potent as caffeine. While theobromine did not significantly increase
any subjective or behavioral measures...when all subjects were combined, the compound increased
alertness, headache, and irritability in some individuals, suggesting
the possibility of individual differences in sensitivity. Using a
higher dose, [one study] found that 700 mg theobromine
lowered blood pressure, decreased self-report calmness and increased
subjects' ratings of how interesting they found performance of study
tasks. (ref)

Your aliens may have biochemistry where theobromine leads to dopamine staying active for longer than normal. Like cocaine does in humans.

The brain’s mesolimbic dopamine system, its reward pathway, is
stimulated by all types of reinforcing stimuli, such as food, sex, and
many drugs of abuse, including cocaine...
Besides reward, this circuit also regulates emotions and motivation.

In the normal communication process, dopamine is released by a neuron
into the synapse (the small gap between two neurons), where it binds
to specialized proteins called dopamine receptors on the neighboring
neuron. By this process, dopamine acts as a chemical messenger,
carrying a signal from neuron to neuron. Another specialized protein
called a transporter removes dopamine from the synapse to be recycled
for further use.

Drugs of abuse can interfere with this normal communication process.
For example, cocaine acts by binding to the dopamine transporter,
blocking the removal of dopamine from the synapse. Dopamine then
accumulates in the synapse to produce an amplified signal to the
receiving neurons. This is what causes the euphoria commonly
experienced immediately after taking the drug. (ref)

In fact, all additive drugs work in similar ways.

All drugs of abuse, from nicotine to heroin, cause a particularly
powerful surge of dopamine in the nucleus accumbens. The likelihood
that the use of a drug or participation in a rewarding activity will
lead to addiction is directly linked to the speed with which it
promotes dopamine release, the intensity of that release, and the
reliability of that release. Even taking the same drug through
different methods of administration can influence how likely it is to
lead to addiction. Smoking a drug or injecting it intravenously, as
opposed to swallowing it as a pill, for example, generally produces a
faster, stronger dopamine signal and is more likely to lead to drug
misuse. (ref)

I see a future of cacao crack and mainline milky ways.

In seriousness though, different species react in different ways to the same substances. The same amount (adjusted by weight) of chocolate that leads to pleasure for humans will kill a dog or a cat.

Theobromine is toxic to a dog when it ingests between 100 and 150
milligrams per kilogram of body weight...It would take 20 ounces of
milk chocolate to kill a 20-pound dog, but only 2 ounces of baker's
chocolate or 6 ounces of semisweet chocolate. (ref)

So, for your aliens, theobromine has an addictive effect. Chocolate being the easiest and tastiest way to get it.

@Cyn has a great answer, and I would add that Theobromine is also being used in toothpastes and other products because of it's unique properties.

Aside from Theobromine, and looking at the manufacturing side of chocolate, I might recommend also:

Ammonium Phosphatide

Ammonium Phosphatide is a type of salt approved for use in the USA as well as the EU. It operates like lecithin (that is, as an emulsifier), and is usually manufactured from rapeseed oil. You can read a detailed description of this in this paper, which focuses on it's use in chocolate production.

Like many types of salts, it's addictive "just like cigarrettes and hard drugs" because of how it interacts with the brain. As such, it is not unfathomable to consider an organism with a brain that interacts with this specific type of salt in a way similar to heroin.