Martha bakes a sweet shower dessert with an audience member.
Preheat oven to 350 degrees, with racks in the middle. Butter two 8-inch round cake pans, and line bottoms with parchment paper rounds. Butter paper and dust both pans with flour, tapping out excess.
In a mixing bowl, whisk together flour, baking powder, and salt.
In the bowl of an electric mixer fitted with the paddle attachment, beat butter and 1 1/4 cups sugar until light and fluffy, about 4 minutes, scraping down sides of bowl as necessary. Scrape vanilla seeds into bowl with a paring knife, reserving pod for another use.
With the mixer on low speed, add dry ingredients in 3 batches, alternating with milk and starting and ending with dry ingredients; scrape down sides of bowl once or twice to ensure proper combining.
In the clean bowl of an electric mixer fitted with the whisk attachment, beat egg whites on low speed until foamy. Add cream of tartar, and beat on medium speed until soft peaks form. Gradually add remaining 1/2 cup sugar, beating on medium-high speed until stiff, glossy peaks form. (Do not overmix the whites; if they appear curdled or broken, the cake will not rise.) Whisk one-third of the whites into the batter to lighten, then carefully fold in the rest of the whites.
Divide batter between prepared cake pans (about 4 cups batter each). Bake until a cake tester inserted in the centers comes out clean and cakes are just beginning to pull away from sides of pans, 30 to 40 minutes. (Do not overbake.) Transfer pans to wire racks and cool for 10 minutes. Run an offset spatula around the edge of each cake, then reinvert cakes onto racks. Remove parchment; cool cakes completely. Refrigerate 1 hour or up to overnight, wrapped well in plastic (or freeze up to 1 month).
Meanwhile, place 4 1/2 cups buttercream in a medium bowl. Add jam and enough food coloring to reach desired shade of pink or blue; stir to combine.
Reinvert cakes and trim tops with a serrated knife to make level. Using a ruler as a guide, insert toothpicks halfway up side of cake, all around. Slice each cake in half horizontally using toothpicks as your guides. Place one cake layer on a piece of parchment or a cardboard cake round. Top with 1 1/2 cups colored buttercream, spreading evenly. Top with another cake layer; repeat with remaining colored buttercream and cake layers, ending with cake. Let set in refrigerator 2 hours.
Frost top and sides of cake evenly with remaining buttercream. Serve.