Heat the oven to 350°F and line a muffin pan with paper liners. The pan should be ready before mixing.
Mix the dry ingredients in a bowl until the color looks even. There should be no cocoa lumps left.
Add the wet ingredients and stir gently until just combined. The batter should look smooth and thick, not runny.
Fill each liner about three-quarters full. The batter should not reach the top.
Bake for 18 to 22 minutes until the tops spring back when touched. The cupcakes should look set and slightly domed.
Let the cupcakes cool fully before adding anything on top. Warm cupcakes will melt frosting.
Frost the cupcakes using a spoon or piping bag. The frosting should sit tall and hold its shape.
Add mini marshmallows on top and drizzle with chocolate syrup. Finish with a light dusting of cocoa powder if desired.