Simply mix your dry ingredients (flour, baking powder, and salt) in a medium bowl. Make sure to mix them a little, so the baking powder gets evenly distributed.

In a large bowl, mix your wet ingredients; sourdough discard, egg, sugar, olive oil, and vanilla extract if you choose to use it. You can mix the wet ingredients with a hand mixer to ensure they are mixed well.

Measure out your blueberries in a small bowl and coat them with some flour.

Slowly add your flour mixture (dry ingredients) to your wet ingredients. I suggest using a hand mixer at first, but the batter will probably thicken too much for the hand mixer to handle it. Finish mixing the batter with a big wooden spoon (or a regular one).

Finally, gently fold in your blueberries. Doing this with a silicone spatula tends to be the most gentle way to fold them in.

You can go both ways here: If you prefer to have muffins that look more purple, mix them in a bit rougher. If you like a yellow muffin, be extra gentle, and blueberries (especially frozen ones) won't release much juice into the batter.

Line your muffin tin (muffin pan or muffin tray) with muffin liners. Spoon the muffin batter into the prepared liners.

You should have just enough batter to fill 12 muffin cups to about ¾ full. Don't worry, the muffins will rise in the oven!

Place the muffin tin into the preheated oven and bake them at 375°F (190°C) for about 20 minutes or until a toothpick inserted in the middle comes out clean.

If you remember to do so, rotate them at the 10-minute mark to ensure they brown evenly.

Wait for 10 minutes or so, then transfer each muffin to a wire rack to cool completely.