Make graham cracker crumbs either in a food processor or by placing graham crackers in a sealed bag and pounding with a rolling pin or another heavy object.
In a medium bowl, combine the graham cracker crumbs, sugar, and melted butter. Mix to combine.
Press the graham cracker mixture into the tart pan. It will feel a little dry, but press firm and it will solidify when baked. If it feels like your crust is too dry, add a little more melted butter (1/2 - 1 Tablespoon more).
Bake the crust until just starting to golden, approximately 10-12 minutes. Remove the crust from the oven.
While the crust is still warm, melt the chocolate chips in a small bowl by microwaving in 15-30 second increments, stirring in between each time, until melted.
Gently pour the chocolate across the crust and spread across the bottom in an even layer using a spatula. Try using single strokes (like you would brush hair) instead of moving the spatula back and forth. Moving back and forth causes more crust crumbs to get into the chocolate. If the chocolate is not spreading well, place the entire tart back into the warm (but off) oven to help melt the chocolate a little more.
Heat the marshmallow fluff for 15-30 seconds in the microwave until just loosened. This will make it easier to spread. Spoon the fluff across the chocolate and with a clean spatula, spread the marshmallow across the tart.
If needed, place the tart back in the warm (but off) oven for the fluff to fill in any gaps, even out, and become easier to spread.
Immediately decorate the tart with swirls of chocolate syrup, mini marshmallows, little graham cracker pieces, and chocolate chips as desired.
Chill the tart until the marshmallow has firmed up, at least 30 minutes. Cut into pieces and serve.