Preheat the oven to 350°F (180°C) then butter and line two 6 inch tins with parchment paper.

In a small bowl, combine the flour, baking powder, and salt. Set it aside.

In a large bowl cream together the butter and sugar with an electric hand mixer or stand mixer. This should take 2 to 3 minutes.

Add in the egg and egg white, mixing well on medium speed until combined.

Pour in the oil, vanilla extract, and milk and mix well.

Add the dry ingredients and mix on medium speed until the batter is formed.

Divide the batter evenly between the two cake pans and bake until golden brown, about 35 to 40 minutes.

Let cool completely, then gently turn the cakes out onto a cooling wrack.

Once the cakes have cooled place them on a cake stand or plate. Using a large palette knife spread an even layer of vanilla buttercream across the first layer of cake. Place the second layer of cake on top and repeat. Using the remaining frosting spread the frosting around the outside of the cake. Do a crumb coat, chill and then repeat the coat with more frosting until the cake is fully covered.

Decorate as you like, I just added sprinkles and a candle. Smash within 48 hours.