To make the frosting: Starting with a double batch of Swiss Buttercream frosting, mix in the honey and vanilla until combined. Set aside. , To make the filling: In a large bowl or the bowl of a stand mixer fitted with the whisk attachment, combine the cream, confectioners’ sugar, raspberry powder, and vanilla. Beat until soft to medium peaks form. Set aside. If your kitchen is warm, store the filling, covered, in the refrigerator until you’re ready to assemble the sheet cake. , To assemble the sheet cake: Place one fully cooled layer of hot milk cake on a large cake board; an upside-down half-sheet pan can work well here. , Use a piping bag to pipe a ¼" to ½"-tall border of frosting (a dam) around the edges of the cake. This will help keep the filling in place. , Use an offset spatula to spread the raspberry preserves or jam over the top of the cake. Keep the preserves within the piped buttercream dam. , Transfer all the filling (you should have a generous 4 cups, roughly 730g) to the center of the cake layer, and carefully spread it on top of the jam to cover the entire surface, all the way to the edges of the buttercream dam. If the buttercream dam isn’t tall enough to contain all the filling, pipe another border on top of the first to keep the filling in place. , Cut the second layer of hot milk cake in half widthwise so you have 2 pieces that measure 13" x 9". (Cutting the cake into 2 smaller pieces makes it easier to assemble.) If your cake layers have been frozen, you can skip this step. , Place the second layer of hot milk cake (either the two cut pieces, side-by-side, or the single frozen layer) on top and gently press to adhere. , Apply a thin layer of buttercream to the top and sides of the cake (a crumb coat), then chill it in the refrigerator for 15 to 20 minutes. Cover the rest of the buttercream and set it aside. , To finish the sheet cake: Remove the sheet cake from the refrigerator and cover the cake with a final, even layer of buttercream. Use a straight-sided tool, like a bench knife, to smooth the sides by holding it at a straight angle against the cake. This will remove excess buttercream and create a smooth finish. , Pipe buttercream borders around the top and bottom edges of the sheet cake, if desired. , Storage information: The cake will keep at a cool room temperature, covered, for several hours. If making ahead (or saving leftovers), store in the refrigerator for up to 1 week, then allow to come to room temperature 1 to 2 hours before serving. Freeze, well wrapped, for up to 1 month.