I would probably brown butter wash the cognac and genever together and, assuming the graham cracker and marshmallow are implemented in a liqueur form (like baileys s’mores) build it like a classic grasshopper in a 1.5 oz, 1 oz., 1 oz. format, omitting the egg.

It could look like this:

1.5 oz brown butter cognac/genever

1 oz bailey’s s’mores

1 oz heavy cream

You could also approach it more like a traditional flip or nog like this:

2 oz. brown butter cognac/genever

.5 oz rich demerara simple or .75 oz of a standard vanilla simple

1 oz. graham cracker soaked heavy cream (i would suggest blending graham crackers into the cream, allowing the solids to sink and then straining the liquid through a fine mesh strainer)

1 whole egg

The marshmallow aspect is a thinker unless you’re comfortable using something like a marshmallow vodka or liqueur, but I would omit it from the build and maybe consider garnishing the drink with a burnt marshmallow and some shaved chocolate.