Preheat the oven to 350°F (175°C). Grease and flour two 9-inch cake pans or spray them with baking spray; set aside.
In a mixing bowl, combine the yellow cake mix, butterscotch instant pudding mix, water, melted butter, and eggs.
Beat on low speed for 30 seconds, then increase to medium and beat for 2 minutes until blended and smooth.
Toss 3/4 cup of the toffee bits with 1 tablespoon flour in a small bowl to coat; gently fold the toffee into the batter.
Divide the batter evenly between the prepared pans and smooth the tops.
Bake for 30 to 35 minutes, until a toothpick inserted in the center comes out clean. Let cakes cool in the pans for 10 minutes, then transfer to a cooling rack to cool completely.
While the cake cools, beat the whipping cream and packed brown sugar together until thick and spreadable to make the whipped topping.
Place one cake layer on a serving plate. Spread just under 1 cup of the whipped cream over the top and sprinkle with 2/3 cup of the remaining toffee bits.
Top with the second cake layer, then cover the top and sides with the remaining whipped cream.
Press the remaining toffee bits onto the sides of the cake and sprinkle a few more on top to finish.