Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or line them with parchment circles.
In a large mixing bowl, combine the cake mix, oil, melted butter, milk, eggs, yolk, sour cream, and vanilla. Mix until smooth and well blended—don’t overmix.
Divide the batter evenly between the pans and bake according to package directions, or until a toothpick inserted in the center comes out clean. Let cakes cool completely before frosting.
For the frosting: Beat the cream cheese and butter until smooth and fluffy. Add powdered sugar gradually, mixing on low. Stir in vanilla, then beat on high for 2–3 minutes until light and airy.
To assemble: Place one cake layer on a serving plate. Spread a generous amount of frosting on top and sprinkle with toffee crunch. Add the second cake layer, then frost the outside of the cake. Pipe a border if desired and finish with more toffee sprinkled over the top.