1. Preheat oven to 350 F. Butter two 6" cake pans and line bottoms with parchment.
2. In the bowl of a stand mixer fitted with the paddle attachment, combine sugar, lemon zest, and juice; mix for a minute or so, until fully incorporated.
3. Add butter; cream until fluffy, 1-2 minutes. Add eggs, one at a time, scraping sides of bowl with a spatula after each addition.
4. Add milk and vanilla, mixing thoroughly.
5. Pour in flour and baking powder and mix until just combined.
6. Divide batter evenly among prepared pans. Bake until toothpick inserted in center comes out clean, about 25-30 minutes. Allow to cool completely.
1. Combine berries, sugar, and lemon juice in a saucepan. Cook over medium heat, stirring constantly, until thickened - about 8-10 minutes. Taste and adjust for sugar as desired.
2. Allow to cool completely; puree in a food processor until relatively smooth.
1. In a heavy-bottomed saucepan, combine sugar and water. Bring to a boil; boil 1-2 minutes, until sugar is dissolved.
2. Remove from heat and pour into a clean container. Whisk in lemon juice. Cool.
1. In a stainless steel bowl, combine sugar and egg whites. Place over a double boiler on medium heat.
2. Cook, whisking frequently, until warm to the touch and sugar is dissolved (it will feel smooth between your fingers). Pour into the clean bowl of a stand mixer fitted with the whisk attachment.
3. Whisk on medium-high until cooled slightly. Add butter, a tablespoon at a time. Continue to whisk a minute or two, until buttercream is smooth and fluffy.
1. Slice each cake in half horizontally. With a pastry brush or spoon, soak three of the layers with the lemon syrup.
2. On the bottom round, spread about 1/3 cup of buttercream in an even layer, followed by an equal amount of blueberry jam. Stack next cake on top and repeat process with next two layers. Add top layer of cake and press gently to adhere.
3. Apply a thin layer of buttercream over entire cake to crumb coat; chill 20-30 minutes.
4. Set about half of remaining buttercream aside. Split the rest in half, dividing into two small bowls. Add one tablespoon of pureed blueberry jam to one bowl; whisk to combine. Add one teaspoon to the other bowl; whisk to combine. You will now have 3 shades of buttercream.
5. Pipe darkest shade of buttercream around the bottom of the cake. Pipe lighter shade next, circling around cake until frosting is gone. Finish with the plain buttercream around the top. Smooth with a bench scraper or spatula.
Cake is best served at room temperature.