Prep your pan and oven. Heat the oven to 350°F (175°C).Grease and line a 9-inch square or 9-inch round cake pan with parchment for easy removal.

Make the crumb topping. In a bowl, mix 1 cup flour, 2/3 cup light brown sugar, 1 teaspoon cinnamon, and a pinch of salt. Stir in 1/2 cup melted butter until clumps form. Chill while you make the batter.

Mash the bananas. In a large bowl, mash 3 very ripe bananas until mostly smooth with a few small lumps.You should have about 1 to 1 1/4 cups mashed.

Whisk in wet ingredients. Add 1/2 cup softened butter, 1/2 cup granulated sugar, 1/4 cup light brown sugar, 2 large eggs, 1/2 cup buttermilk (or 1/2 cup Greek yogurt thinned with 1–2 tablespoons milk), and 2 teaspoons vanilla. Whisk until combined and creamy.

Combine dry ingredients. In a separate bowl, whisk 2 cups all-purpose flour, 1 1/2 teaspoons baking powder, 1/2 teaspoon baking soda, 1 teaspoon cinnamon, and 1/2 teaspoon salt.

Bring the batter together. Add dry ingredients to the banana mixture and fold gently until just combined. Do not overmix—a few streaks are fine. If using, fold in 1/2 cup chopped nuts or mini chocolate chips.

Layer the batter and crumb. Spread half the batter into the prepared pan.Sprinkle with about one-third of the crumb mixture. Add the remaining batter, smoothing the top, then finish with the remaining crumb. This gives you a nice ribbon in the middle and a crunchy top.

Bake. Bake for 38–48 minutes, depending on pan and oven.The cake is done when a toothpick inserted in the center comes out with just a few moist crumbs and the top is golden and set.

Cool. Let the cake cool in the pan for 15–20 minutes, then lift it out using the parchment and move to a rack. Cool until just warm.

Optional glaze. Mix 3/4 cup powdered sugar with 1–2 tablespoons milk or cream and a splash of vanilla. Drizzle over the cooled or slightly warm cake.

Serve. Slice into squares or wedges.Great warm or at room temperature with coffee or tea.