Preheat your oven to 350°F (175°C). Grease a 4-inch round cake pan with butter or cooking spray and line the bottom with parchment paper for easy removal.

Mix the wet ingredients: In a mixing bowl, beat together the softened butter, vegetable oil, and granulated sugar until smooth and creamy. Add the egg and vanilla extract, and continue to mix until combined.

Combine dry ingredients: In a separate bowl, whisk together the all-purpose flour, baking powder, fine salt, and mix well.

Add the flour mixture and milk: Gradually add the dry ingredients to the wet mixture, alternating with whole milk, beginning and ending with the flour mixture. Stir until the batter is smooth and well incorporated.

Bake the cake: Pour the batter into the prepared cake pan. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

Cool the cake: Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.

Beat the butter: In a large mixing bowl, beat the softened unsalted butter with an electric mixer on medium speed until smooth and creamy.

Add powdered sugar: Gradually add the powdered sugar, 1/2 cup at a time, mixing on low speed to avoid a sugar cloud. Once all the sugar is added, increase to medium speed and beat until smooth.

Finish frosting: Add the vanilla extract, heavy cream or milk, and a pinch of salt. Mix until the frosting is light and fluffy.

Once the cake has completely cooled, carefully slice it in half horizontally if you’d like to add an additional layer of frosting inside.

Frost the cake: Spread a generous layer of buttercream frosting on top of the cake, and then gently smooth it out using a spatula. If you sliced the cake, spread frosting in between the layers as well.

Decorate: Finish decorating the cake with sprinkles, fresh fruit, or your preferred toppings.

Serve: Slice, serve, and enjoy your delicious homemade 4-inch vanilla cake!