For the cakes:

1. Preheat the oven to 350°F (180°C). Lightly butter the bottom of three 7-inch (18 cm) cake pans and place a round of parchment on the bottom.

2. In a large bowl, whisk together the flour, baking powder, and salt.

3. In another mixing bowl, whish the eggs, egg white, and vanilla.

4. In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugar on medium speed for about 5 minutes. The mixture should look white in color and fluffy. Reduce the speed to low and slowly stream the egg mixture, scraping down the bowl with a rubber spatula as needed. Mix until combined.

5. Add a third of the dry ingredients to the mixing bowl and mix on low speed to combine. Then slowly stream in half of the buttermilk. Continue alternating dry and wet ingredients, ending on dry, until everything is combined.
6. Fold the sprinkles into the batter with a rubber spatula until just distributed. Do not overmix.
7. Evenly distribute the batter, about 2 large cups (480g), into each of the prepared pans, smoothing out the top. Bake the cakes for about 25 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 5 minutes, then invert the pans to remove the cakes. Transfer the cake to a wire rack and let cool completely.

For the buttercream:

1. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until smooth, about 2 minutes. Add the confectioners’ sugar, and mix on low speed until combined. Increase the speed to medium-high and continue mixing until white and fluffy, about 3 minutes. Add the milk and vanilla and mix on low speed until combined. Then increase the speed to medium and mix again until fluffy.

2. Set aside about 2 ½ cups (75 cl) of the frosting in a medium bowl and cover with plastic wrap. This frosting will be used for decorating the unicorn cake.

Assembling and decorating the cake:

1. If the cakes are not completely flat, slice a thin layer off the tops of the cakes to create a flat surface, using a large serrated knife. Discard the scraps.

2. Assembling. Place one cake right side up on a large plate or cake stand. Using an offset spatula, evenly spread about ½ cup of the frosting over the layer. Add the next layer and spread another layer of frosting. Place the last cake up-side-down, so the top of the cake is smooth and flat. 

3. Crumb coat. Spread a thin layer of frosting, over the top and around the sides as a crumb coat (to help catch any loose crumbs before the final frosting). You should be able to see the cake through the frosting. Chill in the refrigerator for about 20 minutes until the frosting is set.

4. Final frosting. Spread the remaining frosting all over the top and sides, using an offset spatula to smooth out the surface and the sides.

Unicorn decorations:

1. Unicorn cake topping + sprinkles. Garnish the bottom of the cake with extra sprinkles, and add the unicorn eyes while the frosting is still fresh (they won’t fix if the frosting is set and cold). Place the unicorn horn into the center of the cake, and add the small ears on either sides of the horn.

2. Color the buttercream. Divide the remaining buttercream into 4 small bowls. Add one very small drop of food coloring to each (one pink, one purple, one blue, one yellow), and stir to combine. If too pale in color, you can add just a little bit more food coloring. Transfer each preparation into 4 piping bags fitted with star piping tips, and gently squeeze to push the buttercream down to the tip.

3. To pipe the mane, use the colored piping bags to pipe various sized rosettes and dollops on the top of the cake, and in between the cones to fill in any empty space. Then, pipe other rosettes, moving slightly towards one side to make it look like the mane of the unicorn is flowing around the side.

4. Chill the cake (without the horn if too high in the fridge), and remove it from the refrigerator two hours before slicing to allow the cake to come back to room temperature.**** Slice, and enjoy!