Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or grease it lightly with butter.
In a large bowl, whisk together the flour, baking powder, baking soda, and salt. Set this aside for now.
With a stand mixer, combine the sourdough discard, melted butter, sugar, milk, eggs, and vanilla extract.
Next, add the dry ingredients to the wet ingredients and gently stir until just combined. Be careful not to over mix, as this can lead to tough muffins.
If you are using frozen blueberries, add about a tablespoon of flour and let it coat the blueberries so they don't sink to the bottom of the tin.
Gently fold in the blueberries until they are evenly distributed throughout the batter. The burst of sweetness from the blueberries will complement the tangy flavor of the sourdough.
Scoop the batter into the prepared muffin tin, filling each cup about three-quarters full. This will allow the muffins to rise and form a beautiful domed top.
In a small bowl mix ¼ cup of flour and ¼ cup of brown sugar with 3 tablespoons of cold butter and mix with your hands until it is lumpy and crumbles up. SEE NOTES
Place the muffin tin or muffin cups in the preheated oven and bake for approximately 18-20 minutes, or until the muffins are golden brown and for best results, insert a toothpick into the center to make sure it come out clean. The heavenly aroma will fill your kitchen, tempting you to indulge in their warm, tender goodness.
Once golden brown and a toothpick inserted into the center comes out clean, remove the muffins from the oven and let them cool for a few minutes on a wire rack to prevent moisture at the bottom.
Enjoyed with a cup of coffee or with tea!