Crumb topping:
1 cup all-purpose flour
1/3 cup granulated sugar
1/4 cup light brown sugar, lightly packed
1/2 teaspoon kosher salt
1/4 teaspoon ground cinnamon
1/4 pound (1 stick) cold unsalted butter, diced
Cake:
3 cups all-purpose flour
2 teaspoons baking soda
1/2 teaspoon salt
2 sticks (8 ounces) unsalted butter, at room temperature
2 cups sugar
2 teaspoons pure vanilla extract
2 large eggs, preferably at room temperature
About 4 very ripe bananas, mashed (you should have 1 1/2 - 1 3/4 cups)
1 cup sour cream or plain yogurt
1 cup semi-sweet chocolate chips
Center a rack in the oven and preheat to 350 degrees F. Place cupcake liners in two muffin tins.
Make the crumb topping: combine the flour, granulated sugar, brown sugar, salt, cinnamon, and the butter crumb topping ingredients in the bowl of an electric mixer fitted with a paddle attachment. Mix on low speed until the butter is the size of peas. Rub the mixture with your fingertips until it's in big crumbles, then set aside in the refrigerator, tightly covered, until ready to use.
Whisk the flour, baking soda and salt together.
Working with a stand mixer, preferably fitted with a paddle attachment, or with a hand mixer in a large bowl, beat the butter until creamy. Add the sugar and beat at medium speed until pale and fluffy. Beat in the vanilla, then add the eggs one at a time, beating for about 1 minute after each egg goes in. Reduce the mixer speed to low and mix in the bananas. Finally, mix in half the dry ingredients (don’t be disturbed when the batter curdles), all the sour cream and then the rest of the flour mixture. Stir in the chocolate chips with a spatula. Scoop the batter into the cupcake liners and sprinkle each muffin with 1 tablespoon of the crumb topping. Remaining crumb topping can be frozen, well wrapped, for up to two months.
Bake for 20 minutes, or until a thin knife inserted deep into the center of a muffin comes out clean. Transfer the muffin tins to a rack and cool for 10 minutes before moving the muffins to the rack to cool to room temperature.