Begin by preheating your oven to 350°F (176°C). Line three 8-inch round cake pans with parchment paper, and grease the sides thoroughly.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set this aside for later use.

In a large mixing bowl, cream the butter and sugar on medium speed until you achieve a light, fluffy consistency—about 2-3 minutes. Add in the egg whites and vanilla extract, mixing well until everything is fully incorporated. Make sure to scrape the bowl's sides as needed.

Gradually add the dry ingredients and the wet ingredients (milk, lemon juice, and zest) in alternating stages. Begin with about one-third of the flour mixture, followed by half of the milk and lemon mixture, beating on medium speed until everything is combined. Continue alternating between the dry and wet ingredients, ensuring each addition is well mixed. You should end with the dry ingredients.

Distribute the batter evenly among the three pans. Bake the cakes for 25-30 minutes, checking doneness by inserting a toothpick in the center, which should come out clean. If the tops are slightly browned, this is completely normal.

Allow the cakes to cool in the pans for 1-2 minutes before transferring them to a cooling rack to cool completely.

Beat the butter in a large mixing bowl until smooth and creamy. Gradually add half of the powdered sugar, blending until fully incorporated. Then, mix in the salt, vanilla extract, lemon zest, and lemon juice.

Continue to add the rest of the powdered sugar, ensuring a smooth, consistent buttercream. If the mixture seems too thick, add a little more lemon juice to adjust the texture.

In a double boiler or a heatproof bowl placed over simmering water, whisk together the lemon juice, zest, sugar, and egg yolks. Stir constantly until the mixture thickens and reaches 170-180°F, or until it coats the back of a spoon.

Transfer the lemon curd to a bowl and cover the surface with plastic wrap, pressing it directly onto the curd to prevent a skin from forming. Refrigerate until chilled and firm.

Sprinkle the gelatin over the lemon juice in a small bowl and set aside.

In another bowl, whisk the egg yolks. In a saucepan, heat 3/4 cup of heavy whipping cream, sugar, vanilla extract, and lemon zest over medium heat until warm. Slowly whisk this heated cream mixture into the egg yolks to temper them, then return everything to the saucepan. Cook on medium heat until the mixture thickens slightly, about 160°F, and coats a spoon's back.

Stir the gelatin mixture into the warm custard, ensuring it's well combined. Pour the custard into a bowl and place it over ice to cool it to room temperature.

While the custard is cooling, whip the remaining heavy cream and powdered sugar to stiff peaks in a mixer. Gently fold this whipped cream into the cooled custard to create a light, airy Bavarian cream.

Once the cake layers have fully cooled, level the tops if necessary by trimming off any domes. Slice each cake layer in half, creating six layers total. Retrieve the lemon curd and Bavarian cream from the fridge, stirring them until smooth. If needed, give the Bavarian cream a quick whisk to smooth out any lumps.

Start with one cake layer on your serving platter. Pipe a small dam of buttercream around the edges and fill the center with one-third of the Bavarian cream. Place the next cake layer on top. Repeat the process, alternating between Bavarian cream and lemon curd as fillings, until all six layers are stacked.

Apply a crumb coat of frosting to the outside of the cake and refrigerate it for at least 2-3 hours to firm up.

Once chilled, frost the cake with the remaining buttercream, creating a smooth finish. For a decorative, watercolor effect, mix the remaining frosting with a bit of yellow gel color and spread blobs of the tinted frosting across the cake’s surface. Gently smooth it out to achieve a blended look.

Refrigerate the cake until you're ready to serve. For best results, store it in an airtight container and enjoy within 3-4 days.