Step 1: Preheat your oven to 350°F (175°C).
Step 2: In a mixing bowl, cream the softened salted butter and sugar until light and fluffy.
Step 3: Add the large egg and egg yolk to the mixture and mix until fully incorporated.
Step 4: In another bowl, whisk together the all-purpose flour, cornstarch, baking powder, baking soda, and salt.
Step 5: Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Step 6: Gently fold in the regular M&Ms and mini M&Ms.
Step 7: Pour the batter into a greased 9-inch round baking pan and bake for 20-25 minutes.
Step 8: Let the cake cool in the pan for 10 minutes, then transfer to a cooling rack.
Step 9: Prepare the frosting by mixing the softened butter, cocoa powder, powdered sugar, vanilla extract, and milk until smooth.
Step 10: Once cooled, frost the cake and decorate with M&Ms and sprinkles.
