Heat the oven to 350°F (175°C). Lightly grease two 8-inch round cake pans and line the bottoms with parchment paper. In a large mixing bowl, sift together the cake flour, baking powder, and salt, then set aside.

Place the softened butter and caster sugar in a large bowl and beat until the mixture looks light in color and has a fluffy texture, about 4 to 5 minutes. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract until fully combined.

Gradually add the flour mixture, alternating it with the milk in three additions. Add a portion of the flour first and finish with the remaining flour. Stir gently until the batter is smooth and no dry streaks remain.

Pour the batter evenly into the prepared cake pans and smooth the tops. Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean. Leave the cakes in the pans for 10 minutes, then carefully transfer them to a wire rack and allow them to cool completely.

For the frosting, beat the softened butter until creamy and smooth. Slowly mix in the sifted icing sugar, then add the vanilla extract, salt, and heavy cream. Continue beating for 3 to 4 minutes until the frosting becomes light, silky, and easy to spread.

Set one cake layer on a serving plate and spread a generous layer of frosting over the top. Place the second layer on top, then cover the top and sides of the cake with the remaining frosting. Finish the cake with colorful buttercream swirls, rainbow sprinkles, fresh berries, edible flowers, or chocolate decorations. Refrigerate for 20 minutes before slicing to help create clean, even servings.