1. Butter an 8-inch nonstick square cake pan. Cut 2 tablespoons butter into about 1/4-inch chunks and drop evenly in pan. Sprinkle evenly with brown sugar.
2. Slice whole bananas crosswise about 1/2 inch thick. Arrange slices in a single layer in pan. Drizzle evenly with orange juice.
3. In a small bowl, whisk together flour, baking powder, baking soda, cinnamon, and nutmeg.
4. In a large bowl, with an electric mixer on medium-high speed, beat remaining 1/4 cup butter and the granulated sugar until well blended. Beat in egg until well blended, then mashed banana.
5. Add flour mixture and buttermilk alternately, beating on low speed just until completely incorporated. Stir in chocolate chips. Spread batter over bananas in pan.
6. Bake in a 350° oven until a toothpick inserted in center of thickest part (not touching fruit on bottom) comes out clean, 40 to 45 minutes. Cool on a rack about 15 minutes. Run a knife between cake and pan sides. Invert a platter over cake and, holding both together, invert again. Carefully lift off pan. Serve warm.