For the streusel: In a food processor, pulse the granulated sugar, brown sugar, flour, cinnamon, and butter until just combined (the texture of coarse sand or small pebbles). Set aside.

Preheat the oven to 375 degrees F. Line a standard muffin tin with paper liners (if you don’t line them, the muffins will be hard to remove from the pan.

In a large bowl, whisk the flour, baking soda, baking powder, and salt.

In the bowl of a stand mixer with the paddle attachment or in a large bowl using a hand-held mixer, beat the melted butter and white and brown sugars on high speed until well blended. Add the bananas and beat until combined. Add the eggs and beat until combined. Scrape down the sides of the bowl, add the flour mixture and beat on low just until combined, scraping down the sides of the bowl again before giving it a final mix.

Place 1 heaping tablespoon of batter in each muffin cup and top with 1 tablespoon of streusel. Press the streusel lightly into the batter with the back of a spoon, then top with the remaining batter. Sprinkle the streusel topping evenly over the batter.

Bake until the muffins are golden brown and a toothpick inserted into the center of one comes out clean, 18 to 20 minutes.

Let the muffins cool in the pan for 30 minutes, then transfer to a metal rack to cool. 

For the glaze: In a medium bowl, whisk the powdered sugar and milk or water until smooth. Spoon 2 teaspoons of glaze over each muffin. 

Muffins will keep in an airtight container at room temperature for up to 3 days.