Instructions:
Preheat the oven to 350° F.

Combine the flour, sugar, baking powder, and baking soda in a mixing bowl and stir together. Make a well in the center of the flour mixture and pour in the remaining ingredients. Stir together until well blended, but don't over beat. The batter should be medium-thick. If it seems too stiff or dry, add a small amount of additional milk.

Stir in the additional ingredient or ingredients of your choice (see below), then divide the batter among 12 oiled or foil-lined muffin tins.

Bake for 20 minutes, or until the tops of the muffins are golden and a toothpick inserted into the center of one comes out clean. Let cool until just warm to touch, then transfer muffins to a plate. Store them in the refrigerator so they don't get moldy from the moist fruit.

Muffin Variations
Cinnamon Walnut Muffins: Add 1 cup finely chopped nuts, plus ½ teaspoon of cinnamon.
Blue berry Muffins: Add 1 cup fresh or frozen blueberries, plus ½ teaspoon nutmeg, and ½ teaspoon cinnamon.
Strawberry Muffins: Add 1 cup of very sweet chopped fresh strawberries, or the equivalent of drained, frozen strawberries.
Apple Muffins: Add 1 cup of finely chopped apple, and if you like ¼ cup of chopped nuts and 1 teaspoon cinnamon.
Chocolate Chip Muffins: Add ¾ cup of semisweet chocolate chips or mini chips, plus 1 teaspoon cinnamon.
Cheddar Muffins: Use plain yogurt instead of vanilla yogurt and only 2 Tablespoons of granulated Splenda. Add 2/3 cups of cheddar cheese.
Banana Muffins: Add 1 cup of chopped banana or combine ½ cup chopped banana with ½ cup of semisweet chocolate mini-chips.