Adjust oven rack to lower middle position and heat oven to 375 F (or 450 F for mini muffins). Prepare flavourings. Mix flour, baking powder, baking soda and salt in medium bowl; set aside.
Beat butter and sugar with electric mixer on medium-high speed until light and fluffy, about 2 minutes. Add eggs, one at a time, beating well after each addition. Alternating 1/3 at a time, beat in the dry ingredients and yogurt until a smooth, thick batter just forms.
Spray 12-cup muffin tin with cooking spray or coat lightly with butter. Use large ice cream scoop to divide batter evenly among cups. Bake until muffins are golden brown, about 25 minutes for the large muffins (and 12 minutes for the mini muffins). Set on wire rack to cool slightly, about 5 minutes. Remove muffins from tin and serve warm.
Mocha Chip Muffins: Follow Master Recipe, dissolving 3 tbsp instant espresso powder in yogurt and folding 1 cup chocolate chips into finished batter.
Apricot Almond Muffins: Follow Master Recipe, creaming 1 ounce (3 tbsp) almond paste with butter and sugar and folding 1 1/2 cups diced dried apricots into finished batter. Sprinkle each top with portion of 1/2 cup sliced almonds.
Raspberry Almond Muffins: Follow Master Recipe, creaming 1 ounce (3 tbsp) almond paste with butter and sugar. Spoon one-half portion of batter into each muffin cup. With small spoon, make well in center of each cup of dough. Spoon 1 to 1 1/2 tsp raspberry (or other flavor) jam into each well. Fill with remaining batter.
Cranberry-Walnut-Orange Muffins: Follow Master Recipe adding 1 tsp grated orange zest to butter-sugar mixture and folding 1 1/2 cups coarsely chopped fresh or frozen cranberries and 3/4 cup chopped walnuts into finished batter.
Lemon Blueberry Muffins: Follow Master Recipe adding 1 tsp grated lemon zest to butter-sugar mixture and folding 1 1/2 cups blueberries that have been tossed in 1 tablespoon flour into finished batter.
Banana Walnut Muffins: Follow Master Recipe adding 1/2 tsp grated nutmeg to dry ingredients, substituting 1 cup packed light brown sugar for granulated sugar, and folding 1 1/2 cups finely diced bananas (about 3 small) and 3/4 cup chopped walnuts into finished batter.
Lemon Poppy Seed Muffins: Follow Master Recipe adding 3 tbsp poppy seed to dry ingredients and 1 tbsp grated lemon zest to butter-sugar mixture. While muffins are baking heat 1/4 cup sugar and 1/4 cup lemon juice in small saucepan until sugar dissolves and mixture forms light syrup, 3 to 4 minutes. Brush warm syrup over warm muffins and serve.