Rich Chocolate Mayonnaise Cake with Rocky Road Frosting

all-purpose flour ((2 cups))

American cocoa powder ((3/4 cup))

baking powder

baking soda

kosher salt

granulated sugar ((2 cups))

large eggs

full fat mayonnaise ((1 cup))

vanilla extract

espresso powder

boiling water ((1 1/3 cup))

semisweet chocolate chips ((1 cup))

milk chocolate chips ((1 cup))

unsalted butter ((1 stick))

sour cream ((1 cup))

vanilla extract

kosher salt

confectioners' sugar ((2 cups))

mini marshmallows ((homemade or store bought))

toasted mixed nuts ((coarsely chopped))