Mix butter and brown sugar with a stand or a hand mixer until crumbly. Mix in eggs and vanilla until smooth. Mix in salt, baking powder, then flour. The mixture will be wet.

Spread in prepared pan. The mixture is wet so it’s best to spray your hands with cooking spray and then smooth the dough with your hands to get an even layer.

Bake for about 22-25 minutes until they start to turn golden. Err on the side of under baked with these - they’ll finish baking as the cool and you don’t want them over baked. Cool completely before continuing.

Frost with chocolate spread and top with marshmallows. Broil on high for about 1-2 minutes, watching them closely to make sure the marshmallows don’t burn. Once you add the marshmallows, these are best eaten the same day.