Gather all your ingredients on a clean countertop for easy access. Make sure your butter is softened, eggs are at room temperature, and your sourdough discard is ready to use.
In a large mixing bowl, cream together the softened butter and white sugar until light and fluffy. This process incorporates air into the mixture, ensuring a tender cake.
Add in the eggs one at a time, mixing well after each addition until fully combined. Gradually stir in the whole milk and sourdough discard until smooth.
In another bowl, whisk together all-purpose flour, baking powder, and salt if using. Gradually add this dry mixture into the wet ingredients until just combined; be careful not to overmix.
In a small bowl, mix together brown sugar, flour, ground cinnamon, and melted butter until crumbly. Sprinkle half of this mixture over half of your cake batter in your prepared baking dish before adding the remaining batter on top.
Preheat your oven to 350°F (175°C) and bake for about 40 minutes or until a toothpick inserted into the center comes out clean. Allow it to cool slightly before serving warm or at room temperature.