for the cake:
1 & 1/2 teaspoons vanilla
1 teaspoon espresso powder
2 sticks salted butter, melted
1/2 cup dutch-process cocoa powder
4 eggs, room temperature
2 cups sugar
1/4 teaspoon kosher salt
1 & 1/2 cups unbleached, all-purpose flour

for the topping:
1 (10 oz.) bag mini marshmallows
1/2 stick salted butter
1/2 cup milk
1/2 teaspoon espresso powder
1/2 cup dutch-process cocoa powder
4 cups powdered sugar