Preheat the oven to 350℉ and grease an 8-inch square baking pan. Alternatively, you may line the pan with parchment paper for easy removal.
In a medium bowl, stir together the streusel topping ingredients; set aside. In another medium bowl, combine the flour, baking powder, baking soda, and salt; set aside.
Using an electric mixer (stand or handheld with a large bowl), beat the sugar, butter, and egg until fluffy. Beat in the mashed bananas and milk. Add the flour mixture and beat on low to medium speed until just combined.
Spread half the batter into the prepared pan. Sprinkle with half the streusel. Repeat with remaining batter and streusel, and then bake for 40-45 minutes or until just cooked through the center. (Check on the early side since ovens vary. This will prevent overcooking, which leads to dry baked goods. Helpful hint: A quick read thermometer will allow you to nail the perfect level of doneness-the temperature in the center of the cake should read between 200℉ and 205℉. )
Cool cake in pan on rack. This cake tastes best when fully cooled, even though it's tempting to dig right in...which is how I know this!
Notes
Measuring by weight? Perhaps counterintuitively, whole wheat flour weighs less than all-purpose flour. The weight of 1½ cups whole wheat flour is 170 grams. The weight when using 1½ cups of all-purpose flour (or a cup-for-cup gluten-free flour), should be 190 grams. Measuring by volume? Be sure to fluff up the flour before spooning it into the measuring cups-rather than scooping directly into the flour with the measuring cups, which will compress it. Then level the cups off with the straight edge of a knife. Frozen bananas may be used. Simply thaw first and include all juices that seep out during the thawing process.