Preheat the oven to 180°C (160°C fan forced). Grease and line 4 x 20cm round cake pans. Remove the enclosed frosting sachets from the boxes and reserve for another use. Make, bake and cool the cakes as directed on the box using the eggs, oil and water. (See Tip).
Mix 200g of the salted caramel frosting together with the all the vanilla frosting in a bowl. Reserve the remaining salted caramel frosting for the drip.
Level the tops of the cooled cakes, if necessary. Place one sponge on a cake decorating turntable, serving plate or cake stand and spread with 100g (roughly 3 tablespoons) of the caramel and vanilla frosting. Repeat, ensuring each sponge is sitting evenly on top of the one under it. Spread another 350g of the caramel and vanilla frosting in a thin layer over the sides and top of the cake (this is called the crumb coat), leaving the rest of this frosting for the final thicker coat. Chill for 15 minutes until firm to the touch.
Frost the whole cake again with the remaining caramel and vanilla frosting. Use a long palette knife to smooth the sides.
Press most of the popcorn (saving 10g for the top) carefully around the bottom of the cake while the frosting is still soft. Chill the cake again for 15 minutes until firm to the touch. Transfer to a serving plate if not already on one.
For the drip, heat the reserved salted caramel frosting, following the directions on the tub, until quite runny when stirred with a spoon. Working quickly, gently pour the runny frosting onto the centre of the cake and ease over the edges with a spatula, if needed. Drips will form down the sides of the cake (see Tips). Decorate with the honeycomb shards and the remaining popcorn. It's best eaten with 1-2 hours of decorating, but can be kept in the fridge or an airtight container in a cool place for 2-3 days.