Preheat oven to 350 degrees. Spray two cake pans with vegetable spray; line with parchment paper; spray the parchment; and then dust the pan with flour, knocking off excess.
Sift the flour, baking powder, and salt onto a large piece of parchment or into a bowl.
In the bowl of a mixer fitted with a paddle attachment, beat the butter and sugar at high speed until light and fluffy. Add eggs one at a time, and then add the vanilla extract; mix until incorporated.
Turn mixer speed down to low. Add the flour mixture in two additions, alternating with the buttermilk and the lemon zests and juice, scraping down the sides and bottom of the bowl between additions.
Divide batter equally between the two cake pans and bake until cakes are nicely risen and golden, and until a toothpick inserted in the center comes out with just a few crumbs attached.
Remove pans from the oven; let set at room temperature for 20 minutes. Using a small offset spatula, gently loosen any cake that may have stuck to the sides of the pan. Invert cakes onto a wire rack; with another wire rack set atop the inverted cake layers, gently turn cakes right-side up; let cool. Place each layer onto an 8-inch cardboard cake round. Tightly them with plastic and refrigerate until firm, or overnight. Cake layers will keep in the freezer for up to 1 month, tightly wrapped in plastic.
Assemble tiers: Using a turntable, trim each cake top level and split in half horizontally. Set an 8-inch cardboard cake round on the turntable and smear a tablespoon of the black currant buttercream on the board. Set the bottom of one of the cake layers on the board. Brush the layer with a small amount of simple syrup, using a clean 1 1/2-inch-wide pastry brush. Fit a 12- or 18-inch piping bag fitted with a 1/2-inch round pastry tip and fill bag with 2 1/2 cups of the buttercream. Starting from the center of the layer, pipe on a solid disk of buttercream. Set the next cake layer atop the buttercream and brush with simple syrup. Pipe a "dam" of buttercream around the perimeter of the layer. Inside the piping, spoon black currant preserves and smooth with a small offset spatula. Set the next cake layer atop the preserves. Brush this layer with simple syrup, then pipe a disk of buttercream as before. Set the remaining cake layer on top; check to make sure tiers are level and sides are straight (perpendicular to the table). Using a small (or medium) offset spatula, spread a thin coating of the buttercream over tops and sides to seal crumbs. Refrigerate until firm to the touch, about 1 hour. Once cake is chilled, it may be covered with rolled fondant.