Preheat oven to 350 degrees and spray 2 9-inch cake pans with baking spray. (or you can butter and flour them.)
With an electric mixer, beat cake mix, pudding mix, water, melted butter, and eggs on low speed for 30 seconds. Increase to medium speed for 2 minutes.
In a small bowl, combine 3/4 cup toffee bits and flour. Add to batter and stir to combine.
Divide batter evenly between the two cake pans. Bake for 30 to 35 minutes. Cool 10 minutes in pans and then remove from pans and place on a cooling rack.
Beat whipping cream and brown sugar until thick and spreadable.
Place 1 cake layer on a cake stand or serving platter. Spread a little less than a cup of whipped cream on top. Sprinkle with 2/3 cup of toffee bits.
Place second cake layer on top. Cover top and sides with remaining whipped cream.
Press remaining toffee bits onto sides of cake and sprinkle a few on top.