Molasses has a basic viscosity comparable to dense peanut butter. You can often find diluted molasses on the market to make it easier to use, but if you choose a natural, unprocessed product, you should dissolve the molasses with the rum in the shaker by STIRing it before shaking it, otherwise you will leave a lot of undissolved molasses.
Very flavourful rum cocktail. The molasses added to the rum flavours. Did not use the sugar syrup and had it a bit more sour than most may like. Would try this with a float of 151 or with Jamaican rum. Very nice for an evening sipper.
I followed your advice and after trying the Havana 7 year old, I made a second version with Plantation Vintage Jamaica 10 year old (distribued only in Nicolas store in france https://www.nicolas.com/en/Liquors/-Rum-Plantation-Jamaica-10-years-old/p/476383.html)
The difference is indeed appreciable. The cocktail gains a fresher fruit flavour and at the same time a character that is somewhat lacking in the usual Havana 7y. The cocktail has gone from a i try and liked to a favorite.
I tried it with and without the sugar syrup. I thought with molasses and orgeat that sugar was no needed. But it worked a bit better with the sugar. Maybe the same offer could be had on the balance by reducing slightly the amount of lime juice. This is one for the dark-flavours people only. Rum and liquorice.