01 - Preheat oven to 350°F. Grease and flour a 9 by 9-inch square baking pan or line it with parchment paper.
02 - In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
03 - In a large bowl, beat softened butter with granulated sugar until light and fluffy. Add eggs one at a time, then mix in sour cream, milk, and vanilla extract until smooth.
04 - Gradually add dry ingredients to wet mixture, mixing just until combined without overmixing.
05 - Combine light brown sugar and ground cinnamon in a small bowl, stirring to evenly blend.
06 - Pour half the batter evenly into the prepared pan. Sprinkle the cinnamon swirl mixture evenly over this layer, then top with the remaining batter and spread gently to cover.
07 - In a bowl, mix flour, light brown sugar, granulated sugar, cinnamon, and salt. Stir in melted butter until mixture forms coarse crumbs. Sprinkle this evenly over the batter.
08 - Bake for 38 to 42 minutes, until a toothpick inserted in the center comes out clean.
09 - Allow the cake to cool in the pan for at least 20 minutes before slicing and serving.