Add flour, baking soda and salt (and cinnamon, if using) and whisk again until just combined. If you're going to add anything like berries or nuts, fold them in now.
1/2 cup all-purpose flour, 1/2 teaspoon baking soda, pinch salt, pinch of cinnamon, 1/3 cup add ins like berries, chocolate chips, walnuts, etc.
Pour batter into prepared pan (it's best to use the center cups for most even baking), filling cups 3/4 of the way.
Bake for 5 minutes at 425°F, then turn the oven temperature down to 350°F. Bake for another 16-18 minutes, or until a toothpick inserted in the center comes out clean and tops are set and golden.
Let muffins cool in tin for 5 minutes, then transfer to a wire rack to finish cooling.
Video
Notes
Storage: Store the muffins in an airtight container at room temperature for up to 3 days (if they last that long), or in the fridge for a week. To eat warm, pop them in the microwave for 10 seconds, slice, and slather with a tab of butter.