Prepare two half-sheet pans (18×13 inches) by greasing and lining them with parchment paper.
Combine the cake mix, corn starch, sugar, water, eggs, vanilla, and oil in a large mixing bowl. Beat for about 1 minute, or until smooth.
Add the flour, salt, and sour cream. Continue mixing on medium speed for another 2–3 minutes until fully blended and no lumps remain.
Divide the batter evenly between the prepared pans and smooth out the tops.
Bake for 15–18 minutes or until a toothpick inserted in the center comes out clean.
Let the cakes cool at room temperature for about 30 minutes, then chill them in the refrigerator for 30–60 minutes to firm up.
Filling
Beat the marshmallow fluff and softened butter in a mixing bowl on medium-high speed until well combined and fluffy, about 1 minute.
Add the confectioner's sugar, vanilla, salt, and 2 tablespoons of heavy cream. Beat until smooth. If needed, add more cream gradually until the mixture is spreadable but still thick.
Assemble/Decorate
Once the cakes are chilled, use a 2-inch hexagon cookie cutter to cut 36 shapes from the cake layers. To prevent sticking, spritz the cutter with nonstick cooking spray between cuts.
Pipe the filling on half of the cake pieces and top each with another to form a sandwich.
Place the assembled cakes on a parchment-lined tray and freeze them for at least 1 hour or up to a full day. The colder they are, the easier they’ll be to coat.
Melt the almond bark in a heat-safe bowl in 30-second intervals in the microwave, stirring in between, until smooth. Stir in the coconut oil until the mixture is glossy and fluid.
While warm, spoon the melted almond bark over each frozen cake sandwich and allow it to set before flipping the cake and spooning more almond bark over the back side so that it is fully coated. Place the coated cakes on a parchment-covered rack or tray. Let the chocolate set for 10 minutes.
Melt the milk chocolate wafers separately and transfer the melted chocolate to a piping bag with a small round tip.
Finish by drizzling the melted milk chocolate on top in curved stripes to create the classic zebra pattern.