Preheat oven to 350F. Grease and flour three 8" cake rounds and line with parchment.
In a medium bowl, whisk flour, baking powder, and salt until well combined. Set aside.
Using a stand mixer fitted with a paddle attachment, cream butter and lemon zest until smooth. Add sugar and beat on med-high until pale and l (approx 3mins).
Reduce speed and add eggs one at a time fully incorporating after each addition. Add vanilla.
Alternate adding flour mixture with milk & lemon juice, beginning and ending with flour (3 additions of flour and 2 of milk & lemon juice). Fully incorporating after each addition.
Toss blueberries in 1 Tbsp of flour. Gently fold blueberries into cake batter.*
Spread batter evenly between the prepared pans and smooth the tops. Sprinkle with a few additional blueberries if desired.
Bake for approx. 30mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack.
Lemon Syrup:
Place lemon juice and sugar into a small pot. Stir and bring to a boil. Simmer for 1min. Remove from heat and cool completely.
Blueberry Swiss Meringue Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.**
Place bowl over a double boiler on the stove and whisk constantly until the mixture is hot and no longer grainy to the touch (approx. 3mins). Or registers 160F on a candy thermometer.
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment. Slowly add cubed butter and mix until smooth.***
Add vanilla and blueberry powder. Whip until smooth.
Assembly:
Place one layer of cake on a cake stand or serving plate. Poke holes into the cake using a bamboo skewer. Brush with lemon syrup.
Top with approximately 1 cup of buttercream. Repeat with remaining layers and crumb coat the cake. Chill for 20mins.
Frost the top and sides of the cake with remaining frosting. Do dollops on the top of the cake using an Ateco 867 piping tip.
* Batter will be very thick. Be gentle when you fold the berries in so that they don't break.** Ensure there is NO trace of egg yolks in your whites and that your mixer bowl and whisk are completely grease-free or your meringue won't stiffen.*** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth.