Step 1

Preheat oven to 350°F. Grease three, 6-inch pans or two, 8-inch cake pans with baking spray and parchment rounds or homemade cake release.

Step 2

Mix the all-purpose flour, granulated sugar, baking powder, and salt in a large bowl. Use a stand mixer with a paddle attachment or hand mixer to mix on a low speed until combined.

Step 3

Mix the unsalted butter into the dry ingredients on a medium-low speed. Continue to mix until no large chunks of butter remain, and the mixture becomes crumbly.

Step 4

Pour in the egg whites and mix on low until just incorporated and the batter looks wet.

Step 5

Mix in the sour cream, oil and vanilla on a low speed. Scrape the sides and bottom of the bowl as needed with a rubber spatula.

Step 6

If desired, add in gel food coloring at this stage and stir by hand with a rubber spatula until the batter is evenly colored.

Step 7

Divide batter evenly between the prepared cake pans. I like to use a digital kitchen scale to weigh my pans and ensure they all have the same amount of batter. This guarantees the layers will bake to be the same height.

Step 8

Bake for 32-35 minutes or until a toothpick comes out with a few moist crumbs.

Step 9

Allow the pans to cool for 10 minutes, then run a small offset spatula around the perimeter of the pan to separate the cake from the pan.

Step 10

Flip the cake layers onto a wire rack to finish cooling.

Step 11

Use a serrated knife to level the tops of the layers once they’ve cooled completely. If you want to make your cake layers ahead of time, level them then wrap and freeze them.

Step 12

If you make these cake layers in advance and freeze them, let them thaw for about 20 minutes before making your cake. The cake layers should still be slightly cold to the touch, which will make it easier to assemble your cake.

Step 13

While the cake layers bake and cool, make the vanilla buttercream frosting.

Step 14

Beat the unsalted butter on a medium speed for 30 seconds with a paddle attachment, until smooth.

Step 15

Mix in the vanilla extract and salt on a low speed.

Step 16

Slowly mix in 5 cups of powdered sugar on a low speed. Add 2 Tbsp of heavy cream halfway through to make the frosting easier to mix.

Step 17

Continue to mix on low speed for a few minutes, until the desired consistency is reached.

Step 18

If the frosting is too thick, add in additional cream (1 Tbsp at a time). If the frosting is too thin, add in more powdered sugar (quarter of a cup at a time).

Step 19

If you make this buttercream in advance, cover the bowl with plastic wrap to prevent crusting. American buttercream can sit out at room temperature for up to 24 hours or lasts up to a month in the fridge. Just be sure to give the frosting a good stir with a rubber spatula once it thaws to push out any air bubbles and get the consistency smooth again.

Step 20

Stack and frost cake layers on a greaseproof cake board or flat plate.

Step 21

Add an even layer of buttercream between each cake layer with a large offset spatula.

Step 22

Add a thin coat of frosting around the cake, fully cover the cake layers.

Step 23

Smooth using a bench scraper, then chill the cake in the fridge (20 minutes) or freezer (5 minutes) until the frosting is firm to the touch.

Step 24

Add a second, thicker layer of frosting to the cake, and smooth using a bench scraper. Place the cake in the fridge (30 minutes) or freezer (10 minutes) until the frosting is totally chilled.

Step 25

While the cake chills, make the white chocolate ganache drip.

Step 26

Heat the heavy cream in a heat proof bowl in the microwave for 30-45 seconds, until it's steaming and just beginning to bubble.

Step 27

Gently pour the heavy cream over the white chocolate chips, making sure they are fully covered with cream. Allow the mixture to sit for 1 minute.

Step 28

Stir slowly until the cream and white chocolate are combined. Some bits of chocolate may not be fully melted yet. If you notice this in your mixture, heat the mixture again for 15 seconds and stir. Repeat as necessary until all the chocolate bits are fully melted and incorporated.

Step 29

Once the ganache is smooth, mix in the white gel food coloring until the ganache is evenly colored. If you plan to use a plastic bottle to add your drips, pour the mixture into a plastic bottle.

Step 30

Let the mixture cool until the desired viscosity is reached and it's barely warm to the touch.

Step 31

If it seems too thin or thick, you can add a touch more chocolate chips or heavy cream! The type of cream and white chocolate that you use can affect the consistency, so feel free to adjust as needed.

Step 32

Make a test drip with your ganache to see if it's the right consistency, then adjust as needed (i.e. if the ganache is too thin let it cool more or add more white chocolate, or if it’s too thick pop it in the microwave for 5 seconds or add a tiny bit more heavy cream).

Step 33

Once it's the right consistency, add the drips to the chilled cake using a plastic squirt bottle or spoon. After covering the sides of the cake with drips, chill the cake in the freezer for 5 minutes to help the drips set.

Step 34

Add some of the remaining ganache on top of the cake and carefully spread it over the top of the chilled cake.

Step 35

Then decorate as desired! I like to add a fun sprinkle blend around the base of the cake and use the leftover buttercream to pipe dollops on top of the cake using your favorite frosting tip (I used a Wilton 1M).