01 - Preheat oven to 350°F. Grease and flour a 9x9-inch square baking pan or line it with parchment paper.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt for the batter.
03 - In a large bowl, beat the softened butter and granulated sugar until light and fluffy. Add eggs one at a time, then stir in sour cream, milk, and vanilla extract until combined.
04 - Gradually fold the dry ingredients into the wet mixture, mixing just until incorporated.
05 - Combine the brown sugar and cinnamon in a small bowl.
06 - Pour half of the batter into the pan and spread evenly. Sprinkle cinnamon-sugar mixture over the batter, then top with remaining batter and gently spread to cover.
07 - Mix flour, brown sugar, granulated sugar, cinnamon, and salt. Stir in melted butter until coarse crumbs form. Evenly sprinkle over the cake.
08 - Bake for 38 to 42 minutes, or until a toothpick inserted in the center comes out clean.
09 - Allow cake to cool in the pan for at least 20 minutes before slicing and serving.