Prep your pan and oven. Heat the oven to 350°F (175°C). Grease and line an 8-inch square pan or a 9-inch round pan with parchment for easy lifting.

Make the crumb topping. In a bowl, combine flour, brown sugar, cinnamon, and salt. Cut in the cold butter with a fork or your fingers until crumbles form. Stir in nuts, if using. Chill while you make the batter.

Cream the butter and sugar. In a large bowl, beat softened butter with granulated sugar until light and fluffy, about 2–3 minutes. This traps air and helps the cake rise.

Add eggs and vanilla. Beat in the eggs one at a time, then add vanilla. Scrape the bowl so everything is well combined.

Whisk dry ingredients. In a separate bowl, whisk flour, baking powder, baking soda, and salt.

Combine wet ingredients. Stir the sourdough starter and sour cream into the butter mixture until smooth. It may look slightly curdled—this is normal.

Fold in dry ingredients. Add the dry mix to the wet and gently fold just until no dry spots remain. Do not overmix or the cake can turn dense.

Assemble the cake. Spread the batter evenly in the prepared pan. Sprinkle the chilled crumb topping over the surface in an even layer.

Bake. Bake for 32–40 minutes, until a toothpick in the center comes out clean or with a few moist crumbs. The top should be golden and set.

Cool and glaze (optional). Let the cake cool at least 20 minutes. Whisk powdered sugar, milk, and vanilla, then drizzle over the warm cake.

Slice and serve. Enjoy warm or at room temperature. The flavor deepens slightly by the next day.