Preheat your oven to 350°F (177°C) and prepare an 8-inch square pan by lining it with lightly greased parchment paper or greasing it directly.

In a medium bowl, mix together the brown sugar, flour, and cinnamon for the crumb topping. Cut in the chilled butter until the mixture resembles pea-sized crumbs, then set it in the refrigerator.

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

In another large bowl, beat the softened butter and granulated sugar until smooth and creamy, about 3 minutes.

Add the eggs and vanilla to the mixture and beat until well combined.

Stir in the sour cream until you have a lumpy batter.

Blend the dry flour mixture into the wet ingredients without overmixing.

Fold in the milk to create a moist batter.

Spread half of the batter into the prepared pan and sprinkle with half of the crumb mixture.

Dollop the remaining batter over the crumbs and spread it evenly, then top with the remaining crumb mixture.

Bake for about 35–40 minutes until a toothpick inserted comes out clean. Let cool in the pan for 15 minutes.

Whisk together the icing ingredients until smooth and drizzle over the warm cake.

Lift the cake from the pan using the parchment paper, slice into squares, and serve warm or at room temperature.