First, make the streusel: In a medium bowl, combine the sugar, flour, and cinnamon. Cut the butter with a pastry blender until the mixture resembles coarse crumbs. Refrigerate until ready to use.

To make the cake: Preheat oven to 350F. Spray a 9x9-inch baking pan lightly with cooking spray.

In a medium bowl, stir together the flour, baking powder, and salt.

In a stand mixer fitted with the paddle attachment, cream the butter and sugar until light and fluffy, about 4 minutes. Scrape the sides down. Add the egg, milk, and vanilla and mix on low speed until the ingredients start coming together. With the machine running, add the flour mixture, mixing until fully incorporated.

Spread the batter in the prepared baking pan and sprinkle the streusel over the batter.

Bake until a tester inserted in the center comes out clean, 30-35 minutes. Serve warm

Store in an airtight container at room temperature for up to 2 days.