Preheat oven to 400°F. Line 16 to 18 muffin cups with paper liners (see Notes below).
In a large bowl, whisk together the flour, sugar, baking powder, and salt. Work in the softened butter with a fork or pastry cutter. Blend in the eggs, milk, and vanilla until all ingredients are incorporated. Stir in desired add-ins, but do not overmix. Evenly divide the batter between the prepared muffin cups.
Bake for 15 to 20 minutes or until the muffins just test done with a toothpick, taking care not to overbake. After a few minutes, remove the muffins from the pans and cool on a wire rack, or eat them warm out of the oven with a pat of salted butter.
