Pre-heat oven to 350 degrees. Grease and flour two 8-inch cake pans and set aside.
Sift together the cake flour, all-purpose flour, baking powder, baking soda and salt. Set aside.
In the bowl of an electric mixer, beat the butter and cream cheese on medium speed until smooth. Add in the sugar and lemon zest. Continue to mix until light and fluffy – 3 to 5 minutes. With the mixer on medium-low, add in the eggs and egg white, one at a time. Add in the vanilla extract. Stop the mixer and scrap down the bowl.
With the mixer on low, carefully add in half of the dry ingredients. Once incorporated, slowly stream in the milk, then the lemon juice. Add in the remaining dry ingredients. Once incorporated, mix the batter on medium speed for no more than about 30 seconds.
Evenly distribute the batter into the prepared pans. Bake for 28 to 34 minutes, or until a toothpick inserted into the center of the cakes comes out clean. Cool on a wire rack for 10 to 20 minutes before removing the cakes from their pans.
Make the Honey Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.
Place bowl over a pan of simmering water to create a double-boiler. Whisking constantly, heat the egg mixture until it registers 160 degrees on a candy thermometer. Carefully transfer the bowl onto the stand mixer.
Using the whisk attachment, beat the egg white mixture on high speed for 8 to 10 minutes until the bowl is no longer warm to the touch and the meringue is fluffy, glossy and holds a stiff peak.
Switch to the paddle attachment. With the mixer on medium-low, add in the butter, a few tablespoons at a time, and mix until incorporated. Add in the vanilla extract. Turn the mixer up to medium-high and mix until silky smooth.
Add in the honey and mix to combine. At any point does the buttercream appear curdled, just keep mixing. If the buttercream appears soupy, try placing it in the refrigerator for about 10 to 15 minutes then mixing again.
Assemble the Cake:
Once the cakes have completely cooled, carefully slice them in half horizontally with a long, serrated knife. Place the bottom layer on a cake board or serving dish. Spread on about ¾ cup honey buttercream. Slightly hollow out the center of the buttercream with the back of a large spoon. Fill the buttercream “bowl” with about ¼ to 1/3 cup apricot jam. Top with the next layer of cake and repeat.
Crumb coat and then frost the cake with the remaining buttercream.
To decorate as pictured, reserve about 1 cup of the buttercream. Tint it your desired color choice(s). I picked a soft pink and pale golden yellow. For the watercolor finish, frost the cake as normal. Just before the cake is completely smooth, dab on several small spots of color all around and on top of the cake. As you continue to smooth out the cake, the colors will begin to blend together. Add more color as needed. However, the more you smooth, the more blended the colors will become.
Finish off the cake by piping fluffy shell boarders using a medium-large star tip, like a Wilton 1B.
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