1. Make the batter: Preheat oven to 350 degrees F. Using a small brush, lightly coat two 9-inch cake pans with softened butter or vegetable-oil cooking spray. Dust with flour and tap out any excess. Set aside. In a medium bowl, sift together flour, baking powder, salt, and baking soda and set aside. In a large bowl using a mixer set at medium speed, creambutter and sugar until light and fluffy. Add the eggs, one at a time, blending each thoroughly before adding the next. Scrape down sides and beat in vanilla extract. Reduce mixer speed to low and add the flour mixture by thirds, alternating with the lemon juice and buttermilk and ending with the dry ingredients.
2. Bake the cake: Divide the batter equally between the pans and spread evenly. Bake on the middle rack of the oven until the tops spring back when lightly touched and a tester inserted in the center of each cake layer comes out clean -- 20 to 30 minutes. Cool in the cake pans on a wire rack for 15 minutes. Using a knife, loosen the cake layers from the sides of the pans and invert the layers onto the wire rack to cool completely.
3. Frost the cake: Use a serrated knife to trim the cake layers to make them level, if necessary. Line the edges of a cake plate with 3-inch-wide strips of waxed or parchmentpaper and place a cake layer on top. Use a pastry bag or a plastic zip-top bag with a corner cut off to pipe a 1/2-inch-thick ring of Lemon Buttercream around the edge of the cake layer. Spread 1 1/2 cups Blackberry-Cassis Jam inside the buttercream ring and top with the second layer. Place 1 cup of Lemon Buttercream on top of the cake and cover the entire cake with a thin coating of icing. Chill for at least 1 hour. Remove cake from refrigerator and frost with remaining icing. Remove paper strips and serve at room temperature. Store refrigerated for up to 4 days.

Blackberry-Cassis Jam

1. Make the jam: In a medium nonreactive saucepan, combine blackberries, 1 cup cassis, sugar, lemon juice, and lemon zest and cook over medium-low heat until berries begin to soften -- about 10 minutes.
2. Use a slotted spoon to remove only the berries. Place them in a medium bowl, lightly mash them, and strain the resulting liquid back into the saucepan. Set the strained berries aside. Dissolve the cornstarch in the remaining 2 tablespoons of cassis and stir the mixture into the hot berry juice.
3. Bring the liquid to a boil, reduce heat to medium low, and cook, stirring often to avoid burning, until the liquid thickens and is reduced to half its original volume -- about 2 cups.
4. Stir the reserved blackberries into the liquid. Remove from heat and cool completely.

Lemon Buttercream

Make the frosting: In a large stainless-steel bowl placed over a pot filled with 1 inch of simmering water, place the egg whites and sugar. Use a whisk to beat the mixture until it is very hot to the touch (about 160 degrees F). Remove from heat and, using a mixer set on high speed, immediately begin whipping the mixture until it is cool, thick, and glossy and has tripled in volume -- about 5 minutes. Reduce mixer speed to medium and add the butter -- about a quarter cup at a time -- allowing 5 to 10 seconds in between each addition. Add the lemon zest and lemon juice and continue to beat until smooth and fluffy.