1. Preheat your oven to 375 F (190 C). Lightly grease and flour a 9-inch round cake pan, or line it with parchment paper for easy removal.

2. In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt until evenly distributed.

3. In a separate medium bowl, whisk together the sourdough discard, milk, cooled melted butter, egg, lemon zest, and vanilla extract until well combined and smooth.

4. Pour the wet ingredient mixture into the dry ingredient mixture. Stir gently with a spatula or wooden spoon until just combined. Do not overmix; a few lumps are fine.

5. Gently fold in the fresh or frozen blueberries. If using frozen, add them directly from the freezer.

6. Pour the batter evenly into your prepared cake pan. Bake for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean. The top should be golden brown.

7. Let the cake cool in the pan for about 10 minutes before carefully inverting it onto a wire rack to cool completely. While it cools, prepare the glaze by whisking together the powdered sugar and lemon juice until smooth. Drizzle the glaze over the cooled cake just before serving.