Preheat your oven to 350 degrees Fahrenheit and prepare your smash cake mold by lightly greasing it with butter or non-stick spray.

In a small bowl, whisk together the flour, sugar, baking powder, and salt until evenly combined.

In another small bowl, beat the softened butter with a fork or whisk for about one minute until light and fluffy.

Add the egg to the creamed butter and mix well, then stir in the milk and vanilla extract until smooth.

Gently fold the dry ingredients into the wet ingredients using a spatula or fork, stirring just until combined.

Pour the batter into your prepared smash cake mold, filling it about three-quarters full.

Bake for 16 to 18 minutes, until a toothpick inserted into the center comes out clean.

Remove the smash cake from the oven and let it cool in the mold for 5 minutes, then turn it out onto a wire rack to cool completely.

While the cake cools, beat the softened butter in a clean bowl for about one minute until pale and fluffy.

Gradually add the powdered sugar, a tablespoon at a time, beating well after each addition.

Add the cream or milk and vanilla extract, beating until smooth and spreadable.

Once the cake is completely cool, spread or swirl the frosting generously over the top and sides.

Decorate with sprinkles, edible glitter, or fresh berries if desired.