Preheat your oven to 350°F (175°C). This ensures that the muffins bake evenly.

Grease a muffin tin with cooking spray or line it with muffin liners to prevent sticking.

In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, and sugar until well combined. This helps to evenly distribute the leavening agents.

In another bowl, combine the Greek yogurt, milk, vegetable oil, eggs, and vanilla extract. Whisk until smooth. This mixture adds moisture and richness to the muffins.

Pour the wet ingredients into the dry ingredients. Gently fold together using a rubber spatula until just combined. Be careful not to overmix; a few lumps are okay.

If using, gently fold in the blueberries or chocolate chips at this stage.

Spoon the batter into the prepared muffin tin, filling each cup about 2/3 full. This allows room for the muffins to rise.

Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean. This ensures that the muffins are fully cooked.

Remove the muffins from the oven and let them cool in the tin for 5 minutes before transferring them to a cooling rack. This helps them set and makes them easier to remove.

Enjoy warm or at room temperature. These muffins can be stored in an airtight container for up to 3 days.