How to Make It
Preheat oven to 350°F. Beat cake mix, buttermilk, melted butter, egg whites, vanilla, and salt with a stand mixer fitted with a paddle attachment on low speed until ingredients are incorporated, about 1 minute. Increase speed to medium; beat until completely smooth, about 2 minutes.
Coat 3 (9-inch) square baking pans with cooking spray. Spoon one-third of the cake batter into 1 of the prepared pans. Add 4 or 5 red food coloring gel drops to remaining batter; stir with a rubber spatula until fully incorporated. Divide red batter evenly between remaining 2 pans.
Bake cakes in preheated oven until a wooden pick inserted in middles comes out clean, 11 to 13 minutes. Cool in pans 5 minutes. Transfer cakes to a wire rack; cool completely, about 30 minutes.
Place 1 red cake layer on a serving plate; spread with 1/4 cup jam. Top with white cake layer; spread with remaining 1/4 cup jam. Top with remaining red cake layer. Cut cake into 4 equal-size rectangles. Cut each rectangle into 12 (about 1/2-inch) pieces.