• Preheat the oven to 350°F.
• Grease and line, and flour two or three 8-inch round cake pans with parchment paper.
• Whisk the flour, baking powder, baking soda, and salt in a medium bowl. Set aside.
• Beat the sugar and butter together in a large mixing bowl until light and fluffy, about 2–3 minutes.
• Add the eggs one at a time, mixing well after each addition.
• Mix in the lemon juice, lemon zest, and vanilla extract.
• Add the dry ingredients and buttermilk to the butter mixture, alternating each, a little at a time, beginning and ending with the dry ingredients. Mix just until combined.
• Toss the blackberries with 1 tablespoon of flour in a small bowl. Once coated, gently fold them into the batter.
• Divide the batter evenly among the prepared pans and smooth the tops with a spatula.
• Bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean. You can also check doneness by gently pressing the top of the cake. If it springs back, it is done.
• Cool the cakes in the pans for 10–15 minutes, then remove and cool completely on wire racks.

Frosting

• Beat the butter in a large bowl until creamy.
• Slowly add the confectioner’s sugar on low speed.
• Stir in the cream, vanilla, and salt, then increase the speed and beat on medium-high until light and fluffy. Adjust with more cream if needed for spreading consistency.

Assembly

• Place one cake layer on a serving plate or a covered cardboard round.
• Add a thin, smooth layer of buttercream to the top of the cake layer.
• Using a piping bag fitted with a round tip, pipe a continuous ring of frosting around the edge of the cake layer to create a dam for the filling. The dam should be slightly taller than the thickness of the filling you plan to add.
• Spread lemon curd in the center.
• Place the next cake layer on top and then repeat the process with that layer to add the filling.
• Add the top layer and gently press into the frosting of the lower layer so that all the layers nestle together.
• Add a crumb coat of frosting to the entire cake by spreading a thin layer of buttercream, covering all of the cake. Refrigerate for at least 1 hour for everything to firm up.
• Once set, add the final decorative layer of frosting using the remaining buttercream, spreading in a smooth, even layer around the top and sides.
• Decorate with fresh blackberries and lemon slices if desired.
• Chill for 30–60 minutes before slicing for cleaner cuts.