Using a long sharp knife, slice a 13 x 9 inch cake into 2 layers.
Spread a 1/4 to 1/2 inch thick layer of butter cream over the entire bottom cake layer. Spread a generous amount of strawberry jam on top of the buttercream, leaving a 1/2 inch border free of jam. Place the other layer of cake on top.
Dye the buttercream a light tan color. You can use tan food coloring to do this or, if you're using grocery store food coloring, add a the smallest bit of yellow and a touch of cocoa powder to get the color. Frost the entire cake, first with a very thin crumb coat. Chill in the fridge for about 30 minutes. Then do a final coat of buttercream. Spread a slightly darker colored buttercream just around the top edges to give the look of a toasted Poptart.
We used royal icing to frost our cake, as it gave a more realistic resemblance of what a pop tart frosting is. You could simply use buttercream instead if you prefer! To make with royal icing, prepare the icing by combining all ingredients in the bowl of a stand mixer and mixing on medium high for 7 to 10 minutes. Dye royal icing pink and place in a piping bag fitted with a small round tip. Pipe a smaller rectangle with curvy edges in the center of the cake, starting about 2 inches in from the edge. Fill in with royal icing and top with sprinkles.
Using a a skewer or chopstick, poke 12 small holes around the cake, where the tan buttercream is still showing, to give the same texture a pop tart has.