Step 1

Preheat the oven to 350°F. Grease and flour an 8x8x2-inch square baking pan.

Step 2

In the bowl of an electric mixer fitted with the paddle attachment, cream the butter and sugar together on high speed for 3 minutes, until light and fluffy. Scrape down the bowl with a rubber spatula. With the mixer on low, beat in the egg, vanilla, banana, and sour cream and mix until combined. Don’t worry—it may look curdled. In another bowl, sift together the flour, baking powder, baking soda, and salt. With the mixer on low, slowly add the dry ingredients to the wet ones. Scrape into the prepared pan and smooth the top.

Step 3

For the streusel, combine the brown sugar, flour, cinnamon, salt, and butter in a medium bowl and pinch the ingredients together with your fingers until the mixture makes crumbles. Add the chocolate and combine.

Step 4

Distribute the streusel evenly over the batter, sprinkle the almonds on top, and bake for 40 to 45 minutes, until a cake tester inserted in the center comes out clean. Cool in the pan and serve warm or at room temperature.