Preheat oven to 400 F/ 204 C. Prepare muffin tin by greasing around edges and lining cups with cupcake/muffin papers.
Combine dry ingredients in a mixing bowl. Set aside.
Melt butter. Beat in eggs, then add milk. Pour wet ingredients into dry, and mix until just combined.
For 1 cup additions:
Gently mix in your chosen additions, then evenly distribute batter amongst the 12 prepared cups. Bake for 20-22 minutes, until a toothpick inserted into the middle comes out clean.
For Streusel muffins:
Combine the brown sugar, butter, and cinnamon to make the streusel topping. Fill each muffin cup only half full of batter, place a spoonful of streusel in each cup, then top each one with the remaining batter. Spoon the remaining streusel on top of each muffin. Bake for 20-22 minutes, until a toothpick inserted into the middle comes out clean.
Notes
*To make the cooked apples: peel, core, and chop 2 medium apples. Melt 1 Tablespoon butter in a skillet, add chopped apples, 4 Tablespoons sugar, 1 teaspoon of cinnamon, and 1-2 Tablespoons of lemon juice. Cook over medium low heat until apples have softened. Let cool to just warm before adding to batter. (NOTE: Because cooking the apples will create some liquid in the pan, add 1/4 cup more flour to your batter.)