Preheat oven to 350°. Line an 8x8 baking pan with parchment paper.
In a pot, over medium-low heat, melt chocolate and butter.
Remove from heat and stir in eggs, egg yolk, and sugar.
Stir in flour and pinch of salt just until combined. Do not over stir.
Pour batter into your prepared pan.
Cook for 20-30 minutes, until a toothpick inserted comes out clean.
Remove from oven and turn it to low broil.
Top with marshmallows and pop back in the oven for only a few seconds. Do not walk away since marshmallows burn easily.
Let cool and cut into squares.