Preheat the oven to 350 degrees.
Grease and flour your pans. This recipe will fill two, 8" round pans that are 2" deep.
Using your mixer, beat the butter until smooth. Add in the granulated and brown sugar and beat on low until combined, then on medium high for a couple minutes.
Add in the eggs, one at a time and mix on low/meium after each addition, just until combined. Do not over mix.
In a separate bowl, mix together the flour, baking powder, baking soda, salt and cinnamon. Whisk to mix it a bit.
In another bowl, mix together the sour cream, whole milk, mashed bananas and vanilla. Whisk well.
Add your dry ingredients and your liquid ingredients alternately to the egg and sugar mixture. Start with a third of the dry ingredients, mix on medium after that addition, just until combined. Then add half of the liquid ingredients and mix on medium just until combined. Continue on with another third of the flour mixture. Mix just until combined. Add the last half of the liquid ingredients and mix. End with the last third of the dry mixture. It's important to only mix until each addition is combined. Do not over mix.
Fold in a cup of chopped toffee.
Pour into prepared pans.
Bake at 350 degrees for approximately 45 - 50 minutes. Start checking at 40 minutes.
Remove from oven and set on racks to cool for 10 minutes. Turn out the cakes onto racks after 10 minutes to cool completely.
Ice with browned butter toffee buttercream.