• Preheat oven to 350°.
• Spray a 13-x-9-inch baking pan with nonstick baking spray with flour. Set aside.
• In a medium bowl, combine flour and baking powder, whisking well. Set aside.
• In a mixing bowl, combine butter and both sugars. Beat at high speed with an electric mixer until mixture is creamy, approximately 5 minutes. Add eggs, one at time, beating to combine. Add molasses and vanilla extract, beating well. Add flour mixture, and beat at low speed until flour has been incorporated, approximately 2 minutes, scraping down sides of bowl as needed. Add walnuts, stirring until incorporated.
• Spread batter into prepared pan, smoothing level with an offset spatula. Tap pan sharply on countertop several times to release any air bubbles.
• Bake until a wooden pick inserted near center comes out clean, approximately 25 minutes. Let cake cool in pan for 10 minutes.
• Place a wire rack over cake pan, and invert cake onto rack. Let cool completely.(For decorating purposes, bottom of cake will be top of cake.)
• Using a long, serrated knife, trim ends from all sides of cake. Cut 15 (2¼-inch) squares from cake. (Use grids of wire cooling rack to aid in cutting squares.) Cut each cake square in half horizontally to create 30 cake layers.
• Place Toffee Buttercream in a piping bag fitted with a medium (Wilton #21) open-star tip.
• Pipe a border of buttercream onto cut sides of 15 bottom layers.
• Place a top layer of cake on each bottom layer, cut sides together. Pipe a decorative buttercream border around edges of top layer of each cake.
• Using an offset spatula, carefully and evenly spread melted apricot jam inside piped buttercream border.
• Garnish with apricot slices, if desired.
*We used King Arthur Self-Rising Flour.
MAKE-AHEAD TIP: Unfrosted cake squares and buttercream may be made a day in advance. Store cake squares at room temperature in an airtight container. Refrigerate buttercream in an airtight container. Let it come to room temperature before using.