Mix dry ingredients In a bowl, whisk together flour, baking powder, salt, cocoa powder, and espresso powder. Set aside.

Make butter mixture In a small pot over medium heat, melt butter, sugar, and oil together until fully melted and smooth. Set aside to cool slightly.

Mix wet ingredients In a large bowl, whisk eggs, egg yolk, vanilla, and sugar until well combined.

Combine mixtures Slowly add half of the butter mixture to the wet ingredients and mix. Then add the remaining half and mix again.

Add dry ingredients Fold in the dry ingredients until just combined (do not overmix).

Bake Pour into a greased 9x9 inch pan. Bake at 350°F (177°C) for 25–30 minutes.

Add marshmallow topping Immediately after baking, top with marshmallows.

Broil Broil on high for 2–3 minutes until marshmallows are golden and toasted (watch closely!).