Prep: Preheat Oven to 350˚F. Line bottoms of two 9″ cake pans with parchment paper (do not grease the sides).
How to Make the Cake:
In the bowl of an electric stand mixer fitted with whisk attachment, beat 6 large eggs 1 min on high speed. With the mixer on, gradually add 1 cup sugar and continue beating 8-10 min until thick and fluffy.
Whisk together 1 cup flour and 1/2 tsp baking powder then sift this mixture into fluffy egg mixture one third at a time. Fold with a spatula with each addition just until incorporated. Scrape spatula from the bottom to catch any pockets of flour and stop mixing when no streaks of flour remain. Do not over-mix or you will deflate the batter.
Divide evenly between prepared cake pans and bake at 350˚F for 23-28 minutes (my oven took 25 min), or until top is golden brown. Remove from pan by sliding a thin spatula around the edges then transfer to a wire rack and remove parchment backing. Cool cakes to room temp then slice layers equally in half with a serrated knife.
How to Make Lemon Blackberry Frosting:
Combine 2 Tbsp lemon juice with 1 cup blackberries in small saucepan. Simmer 6-7 minutes or until berries easily mash up with a fork then turn off heat. Strain through a fine mesh strainer, pushing the blackberries through with a spatula until only seeds are left in strainer. Discard seeds and set the puree aside to cool completely to room temp (you should get 5 Tbsp puree).
In the bowl of an electric stand mixer fitted with whisk attachment, cream together 3 sticks butter, 3 cups powdered sugar and 1/2 tsp salt on low speed until combined, scraping as necessary. Increase speed to medium-high and beat until white and fluffy (2 to 3 min).
Add cream cheese 1-piece at a time and mix until combined then continue to beat 2 min until fluffy, scraping down the bowl as needed. With the mixer on, drizzle in blackberry syrup 1 Tbsp at a time then beat another minute.
How to Assemble Blackberry Cake:
Slice 1 cup blackberries and set aside. Make your lemon syrup by combining 3/4 cup warm water, 1 Tbsp granulated sugar and 4 Tbsp Lemon Juice.
Place first cake layer on serving platter cut-side up **. Brush with 1/4 of the lemon syrup. Spread frosting over the top and layer with 1/3 sliced berries. Repeat with remaining layers placing the final layer cut-side-down, then frost the top and sides.
Pipe remaining frosting onto cake. For the roses, use a closed star Wilton 2D tip. Decorate with remaining blackberries and sprinkle sugar crystals if desired.