Step 1

Preheat the oven to 350˚F (180˚C).

Step 2

Grease a 13 inch x 9 inch (32.5 centimeter x 23 centimeter) baking tin and line with baking paper.

Step 3

To make the crumb topping, combine the flour, sugar, cinnamon, and salt in a bowl. Pour in the melted butter and stir until well combined. Set aside.

Step 4

Using a stand mixer or electric beaters, beat the butter and superfine sugar in a large bowl until light and creamy.

Step 5

Add the eggs, one at a time, beating until well combined after each addition.

Step 6

Add the sour cream, milk, and vanilla extract and beat to combine.

Step 7

Sift the flour, baking powder, baking soda, and salt into a bowl and stir to combine.

Step 8

In batches, add the flour mixture to the liquid ingredients, beating until just combined.

Step 9

Transfer the batter to the prepared tin and smooth the top.

Step 10

Using your hands, squeeze small handfuls of the crumb topping to compact it, then crumble into smaller and larger clumps over the batter. Press down gently to lodge the clumps in the batter and cover the top completely.

Step 11

Bake for 50-60 minutes, until a skewer inserted into the center comes out clean.

Step 12

Leave to cool in the tin, then dust generously with confectioners' sugar.

Step 13

Cut into rectangles to serve.