Method — Cake

1. Preheat the oven to 350°F (175°C). Grease and flour two 8-inch round cake pans (or line with parchment).

2. In a bowl, whisk together the cake flour, baking powder, baking soda, salt, and cinnamon; set aside.

3. Using a stand mixer or hand mixer, cream the softened butter with the granulated and brown sugars until the mixture lightens slightly.

4. Add the eggs one at a time, mixing briefly after each addition until incorporated. Stir in the vanilla. Avoid overworking the batter.

5. In a separate vessel, combine the sour cream, milk, and mashed bananas until smooth.

6. Add the dry ingredients and the banana mixture to the creamed sugar in alternating additions, beginning and ending with the dry ingredients. Mix only until the ingredients are just combined.

7. Gently fold in the chopped toffee.

8. Divide the batter evenly between the prepared pans and bake for about 45–50 minutes, beginning to check at 40 minutes. A toothpick inserted in the center should come out with a few moist crumbs.

9. Cool the cakes in the pans for 10 minutes, then invert onto wire racks and cool completely before frosting.

Method — Browned-Butter Toffee Buttercream

1. In a light-colored saucepan, melt the combined butters over medium heat. Cook, stirring frequently, until the milk solids turn golden and you smell a toasty, nutty aroma. Remove from heat immediately to avoid burning.

2. Stir the chopped toffee into the warm browned butter so the bits soften and infuse flavor. Allow this mixture to cool completely — chilling speeds this up. Cooling is important so the frosting whips correctly.

3. Once the browned-butter mixture is cool and slightly firm, place it in a mixing bowl and beat until creamy. Add the vanilla.

4. On low speed, add about half the powdered sugar, then raise the mixer and continue adding the remaining sugar with 3 tablespoons of milk. Beat until smooth, adding extra cream a teaspoon at a time if the frosting is too stiff, until you reach a spreadable consistency.