Step 1

Preheat the oven to 325F. Butter an 8x8-inch square baking pan or spray with baking spray. Line with parchment paper.

Step 2

In a large bowl, whisk together ¾ cup flour, sugars, cinnamon, and salt. Using a fork, stir in the melted butter until mostly combined but still crumbly. Stir in the remaining 1/2 cup flour until the mixture is very crumbly and still has large pieces.

Step 3

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

Step 4

In a large mixing bowl, combine the butter and sugar and beat on medium speed until light and fluffy, about 3 minutes. (Use the paddle attachment if using a stand mixer.) Add the egg and beat until combined. Scrape down the bowl. Add the sour cream, milk, and vanilla, and beat on medium-low speed until combined. (The mixture will look runny and curdled.)

Step 5

With the mixer on low speed, slowly add the flour mixture to the butter mixture. Beat just until all of the flour is incorporated, stopping to scrape down the bowl as needed. Transfer the batter to the prepared pan and spread it evenly. (Batter will be very thick.) Sprinkle the crumb topping all over the batter.

Step 6

Bake for 40 minutes or until a toothpick inserted into the center comes out with a few moist crumbs. If the cake is browning heavily before 40 minutes, loosely cover with foil. Let the cake cool completely in the pan. Sprinkle with confectioners’ sugar before slicing and serving.