Preheat oven to 350 degrees F (175 degrees C). Generously grease and flour a 9-inch fluted tube pan (such as a Bundt(R)).
Mix butter pecan cake mix, water, vegetable oil, eggs, bananas, pecans, and toffee baking bits in a bowl until thoroughly combined. Pour batter into prepared tube pan.
Bake in the preheated oven until cake is lightly golden brown and a toothpick inserted into the center comes out clean, 30 to 40 minutes. Cool the cake in the tube pan for 15 minutes. Place a serving platter on top of the pan and invert pan. Gently lift pan from cake and let cake finish cooling.
Melt butter and brown sugar in a saucepan over low heat, stirring until combined. Bring to a boil, stir in milk, and return to a rolling boil. Remove from heat; beat confectioners' sugar and vanilla extract into butter mixture to make a smooth frosting. Spread over cooled cake and allow to stand until frosting has set, about 15 minutes.
