Preheat your oven to 350°F (175°C). Prepare a 4-inch cake pan by lining it with parchment paper.

Cream together softened butter and sugar until light and fluffy, about 3-4 minutes. Mix in the egg and vanilla extract.

In a separate bowl, whisk together the flour, baking powder, and salt, then gradually add to wet ingredients along with the milk.

Pour prepared batter into the pan, smooth the top, and bake for around 22-25 minutes until a toothpick comes out with a few moist crumbs.

Let the cake cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.

Level the top of the cooled cake with a serrated knife and slice it horizontally into 2-3 layers.

Beat softened butter with powdered sugar, vanilla extract, and heavy cream until light and fluffy.

Spread frosting between each cake layer, apply a crumb coat, chill for 15 minutes, then frost completely.