Preheat oven to 400°F Coat a 12-cup muffin pan with cooking spray, or line with paper liners.
In a bowl, combine flour, baking powder, sugar, and salt. In a large bowl, beat eggs, milk, oil, and vanilla for 1 minute on medium. Add the flour mixture and beat just until no streaks of flour remain -- do NOT overmix!
Spoon batter into muffin cups. Bake for 18-20 minutes or until tops of muffins spring back when pressed lightly. Transfer muffins to a rack to cool slightly.
CINNAMON SUGAR MUFFINS: Sprinkle muffin tops with a mixture of 3 tbsp sugar and 1 tsp cinnamon before baking.
BLUEBERRY STREUSEL MUFFINS: Stir 1 C blueberries into batter. Before baking top muffins with streusel: Combine 1/4 C flour, 1/4 C brown sugar, and 2 tbsp cold cubed butter.
OATMEAL RAISIN MUFFINS: Stir 1 C raisins into batter and sprinkle muffin tops with rolled oats.
RASPBERRY ALMOND MUFFINS: Fill muffin cups a third of the way with batter, then spoon 1 tsp seedless raspberry jam onto each. Top with more batter and sprinkle with slivered almonds before baking.
BANANA CHOCOLATE CHIP MUFFINS: Stir 1 mashed banana and 1/2 C chocolate chips into batter, then bake.
LEMON POPPYSEED MUFFINS: Substitute 1 tsp lemon extract for the vanilla and add 2 tbsp poppy seeds to the batter, then bake.
APPLE PIE MUFFINS: Mix in 1 tsp cinnamon, 1/2 nutmeg, and 1/2 to 1 C chopped apple into the batter, then bake.
PINEAPPLE COCONUT MUFFINS: Stir 1/2 C shredded coconut and 1/2 C drained, chopped, canned pineapple into batter. Sprinkle each muffin with 1 tsp coconut before baking.