Preheat your oven to 375°F (190°C) and grease a 9x13 inch baking pan with butter or cooking spray.

In a medium bowl, combine brown sugar, flour, and ground cinnamon until well blended. Add melted butter and mix until crumbly.

Cream together softened butter and white sugar until light and fluffy, about 3-5 minutes.

Mix in vanilla extract, then add eggs one at a time. Finally, pour in milk and sourdough discard or starter, mixing until well combined.

Sprinkle baking powder and all-purpose flour over the wet mixture and mix gently until just combined.

Pour half of the batter into the prepared baking pan and smooth it out.

Sprinkle half of the cinnamon sugar mixture over the first batter layer.

Pour the remaining batter over the cinnamon sugar layer and smooth out.

Sprinkle the last of the cinnamon sugar mixture over the top of the batter.

Bake for 40-45 minutes until golden brown and a toothpick comes out clean.

Let the cake cool in the pan for 10-15 minutes before slicing and serving warm.