Move the oven rack to the middle position. Preheat the oven to 425°F (220°C). Grease two muffin pans with nonstick spray or line them with muffin papers. Set aside.
In a large bowl, whisk together the 2 ½ cups all-purpose flour, 2 tablespoons cornstarch, 1 tablespoon baking powder, ½ teaspoon baking soda, and ½ teaspoon kosher salt until well combined.
Add the 2 cups raspberries to the flour mixture. Toss them gently to coat. Coating the cranberries in flour will help keep them from sinking to the bottom of the muffins.
In a separate bowl, add the 1 ½ cups granulated white sugar, 1 cup sourdough discard, ⅔ cup neutral oil, ½ cup buttermilk, 3 large eggs, , and 1 tablespoon vanilla. Mix until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined. Be careful not to overmix; a few lumps are fine.
Divide the muffin batter evenly among the muffin cups filling nearly to the top.
In a small bowl, add the ½ cup all-purpose flour, ½ cup granulated white sugar, ¼ teaspoon kosher salt, and 4 tablespoons unsalted butter. Use a fork to cut the butter into the other ingredients until the mixture is crumbly and the butter pieces are no larger than a pea.
Sprinkle the streusel over each muffin.
Bake the muffins in the preheated oven for 5 minutes, then, keeping the muffins in the oven, reduce the oven temperature to 350°F (180°C) and continue to bake for 13 to 16 minutes until the tops are golden brown.
Allow the muffins to cool for several minutes in the muffin pan before transferring them to a wire rack to cool completely.
Notes
Storage: These muffins will stay moist for a couple of days when stored at room temperature. Leftover muffins can be stored in an airtight container for 2-3 days.For longer storage keep them in the refrigerator for up to a week.They can also be frozen for up to 3 months. Wrap each muffin in plastic wrap and place them in a zip-top bag. Thaw muffins at room temperature.Tips
Muffins bake best on the middle rack of the oven. Too low and the bottoms will burn, too high and the tops will brown too quickly.
If you are using frozen raspberries, there is no need to defrost them first.
Make sure the raspberries are coated with the flour mixture before adding the wet ingredients. This will ensure they are evenly distributed throughout the batter and they won't sink to the bottom.
Don’t over-mix the batter. A few lumps are fine. Overmixing will create too much gluten causing your muffins to be tough.
Use a cookie scoop to evenly portion the cupcake batter into the muffin pans.
Use a toothpick to test for doneness. Insert a toothpick into the center of a muffin, if it comes out clean, the muffins are done.