Oatmeal-Banana Coffee Cake

Preheat the oven to 375 degrees F (190 degrees C). Spray a 10-inch square baking pan with cooking spray.

Combine milk and vinegar in a glass to make sour milk; stir gently. Mash bananas in a small bowl to make 1 cup.

Combine all-purpose flour, whole wheat flour, oats, baking powder, baking soda, and salt in a bowl.

Cream butter and brown sugar in a large mixing bowl with an electric mixer until fluffy. Beat in eggs and vanilla extract. Mix in bananas. Add dry ingredients in 2 batches, alternating with sour milk; mix thoroughly. Pour batter into the prepared pan.

Mix brown sugar, flour, and butter for topping together in a bowl. Sprinkle topping over batter, then sprinkle on pecans.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 25 to 30 minutes. Cool on a wire rack before cutting into squares.