1. In a medium bowl, combine 1 1/4 cups all-purpose flour, 1/2 cup light brown sugar, 1/2 cup granulated sugar, and 1 tablespoon ground cinnamon. Add 1/4 cup melted unsalted butter and mix until large, moist crumbs form. Set aside.

2. In a large bowl, whisk together 1 cup all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/2 teaspoon salt.

3. In a separate bowl, beat 1/2 cup softened unsalted butter and 1/2 cup granulated sugar until creamy. Add 2 large eggs one at a time, mixing well after each. Blend in 1 cup sour cream, 1/4 cup whole milk, and 1 1/2 teaspoons vanilla extract.

4. Add dry ingredients to wet and mix just until combined. Spread batter evenly into a greased 9×13-inch baking pan. Sprinkle crumb topping evenly over the batter.

5. Bake at 350°F (175°C) for 35-40 minutes, or until a toothpick inserted into the center comes out clean.

6. Cool cake in the pan for at least 20 minutes. In a small bowl, whisk 1 cup powdered sugar, 1 1/2 tablespoons milk, and 1/2 teaspoon vanilla extract until smooth. Drizzle glaze over cooled cake before serving.