Make the streusel: stir 1/2 cup flour, brown sugar, and cinnamon, then cut in the cold butter until crumbly.

Cream the butter and sugar until fluffy, then beat in the egg, vanilla, and sourdough discard.

Whisk the remaining dry ingredients together, then add to the wet mixture in three additions alternating with the milk, mixing just until combined.

Spread half the batter in a greased 8-inch square pan, top with half the streusel, and repeat layers. Bake at 350°F (175°C) for 35 to 40 minutes.

Cool in the pan for 10 minutes, then transfer to a rack. Slice and serve warm or at room temperature.