Heat oven to 350°F/180°C. Butter and line three 6-inch cake pans with parchment paper.

In a large jug combine and whisk together the egg whites, whole egg, milk, and the vanilla extract. Set aside.

In the bowl of an electric mixer fitted with the paddle attachment, combine the dry ingredients together at low speed for 1 minute. (You can also do it by hand.)

Add the cold butter and continue to mix on low until the mixture is a fine crumbly texture.

Add the liquid mixture into your dry ingredients in 2 separate batches making sure to scrape the sides and bottom before and after each batch, mixing until light, fluffy, homogenous, and well incorporated. Fold once or twice to ensure the batter at bottom of the bowl is incorporated.

Divide the batter evenly into the prepared pans.

Bake until a cake tester comes out with a few crumbs when inserted into the center, about 25-35 minutes. Be very careful to not over-bake. Check cake at 25 minutes to see how it is doing and judge the timing from there.

Once baked, let the cakes cool on racks for 20 minutes before removing them from the tins.

When the cake is completely cold you can start to decorate. If you are not using it straight away wrap the cake tightly and store it at room temperature for up to 2 days, refrigerator for up to 5 days, or frozen for up to 2 months.

To Assemble the Cake: With a serrated knife cut off the top of the cake layers to give them even, level surfaces to frost on.

Place the bottom cake layer on the table or cake turntable.

Spread your chilled buttercream frosting on top with a small offset spatula. Gently place 2nd cake layer face down on top. Place more frosting on top of this layer and spread out.

Gently add the 3rd cake layer. Add a generous scoop of frosting and spread evenly with a small offset spatula working your way down the sides until you have a thin layer of frosting over the entire cake. This is called the “Crumb layer”. Chill until set, about 30 minutes.

Remove from refrigerator and apply a final “coat” of frosting. Decorate all over with sprinkles. Just keep on applying them until they stick all over.

Store in the fridge for up to 3 days. Before serving let it at room temperature for 30 minutes and then enjoy!