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For the banana cake: Preheat the oven to 325 degrees F. Line a 12-by-18-inch jelly roll sheet pan with parchment and spray with nonstick cooking spray. Line a tray or baking sheet with parchment.
Heat the butter in a medium saucepan until browned. You will see tiny rust-colored specks float to the top and the butter may become foamy when it is ready. Remove from the heat and let cool slightly.
Whisk together the pastry flour, baking powder, salt and baking soda in a large bowl and set aside. Add the brown sugar and eggs to the bowl of an electric mixer fitted with a whisk attachment and mix on medium speed for 30 seconds. Whisk the brown butter, bananas, sour cream and vanilla together in a medium bowl, then add to the egg mixture and mix until just combined. Add the dry ingredients and mix on low speed until just combined.
Pour the batter into the prepared jelly roll sheet. Bake, rotating the pan 180 degrees halfway during cooking, until the cake is light golden brown and a skewer inserted in the center of the cake comes out clean, 15 to 18 minutes.
Cool the cake completely and freeze for 1 to 2 hours. Turn out the frozen cake onto a flat surface and use a 2 1/2-inch round cookie cutter to cut out 24 even circles. Place the rounds on the prepared tray and cover until ready to assemble and frost.
For the milk chocolate frosting: Add the butter to the bowl of an electric mixer fitted with a paddle attachment and cream on slow speed until smooth, glossy and free of lumps (do not fluff).
Add the milk and vanilla and mix to incorporate.
Add half of the confectioners' sugar and mix to incorporate. Add the rest of the sugar and mix on low speed until smooth.
Add the milk chocolate and bittersweet chocolate and mix on medium speed until fully incorporated, scraping down the bowl as you go.
For the toffee: Preheat the oven to 325 degrees F. Place a clean silicone mat on another 12-by-18-inch sheet pan. Combine the sugar, corn syrup and cream in a heavy-bottomed saucepan. Bring the mixture to a boil over medium heat without stirring. Add the butter and begin to stir until the caramel turns the color of peanut butter.
Pour immediately onto the prepared baking sheet and bake until the toffee turns a deep amber color and spreads thinly on the pan, 10 to 12 minutes. Let cool, then chop into tiny pieces that will be easy to chew.
To assemble: Fill a pastry bag with some of the milk chocolate frosting.
Top 2 mini cake rounds with frosting and about 2 teaspoons of chopped toffee. Stack these on top of each other and top with an unfrosted round of cake. Wrap the small cylinder in plastic wrap. Repeat with the remaining cake rounds. Freeze until the frosting has set, about 30 minutes.
Place a small cake on a revolving cake stand and pipe on the frosting to cover completely. With a small offset spatula, even out the surfaces of the little cake and swirl the frosting in a pretty, decorative pattern of your choice.
Top each mini cake with crushed toffee and dust with confectioners' sugar.