Vanilla Cupcakes: Preheat oven to 350 degrees F (180 degrees C) and lightly butter or line 12 muffin cups with paper liners.

In the bowl of your electric mixer, or with a hand mixer, beat  the flour, sugar, baking powder, and salt until combined. Add the butter, eggs, vanilla extract, and sour cream (or yogurt). Beat the wet and dry ingredients together, at medium speed, until the batter is smooth and satiny, about 30 - 60 seconds. Scrape down the sides of the bowl as needed.

Evenly fill the muffin cups with the batter and bake for about 20 - 25 minutes or until lightly browned and a toothpick inserted into a cupcake comes out clean. Remove from oven and place on a wire rack to cool. 

Once the cupcakes have cooled completely, cut about an 1/2 inch (1.5 cm) hole into the center of each cupcake, using a melon baller or a small spoon. Then spoon (or pipe) a heaping teaspoon of lemon curd into each indentation.

Lemon Curd: In a stainless steel bowl, placed over a saucepan of simmering water, whisk together the eggs, sugar, and lemon juice until blended. Cook, stirring constantly with a heatproof rubber spatula or wooden spoon (to prevent it from curdling), until the mixture becomes thick (like sour cream or a hollandaise sauce) (160 degrees F or 71 degrees C). This will take approximately 10 minutes. Remove from heat and immediately pour through a fine strainer to remove any lumps. Cut the butter into small pieces and whisk into the mixture until the butter has melted. Add the lemon zest and let cool. The lemon curd will continue to thicken as it cools. Cover immediately (so a skin doesn't form) and refrigerate until cold (preferably overnight). The lemon curd can be made several days (up to a week) in advance.

Lemon Flavored Whipped Cream: Place your mixing bowl and wire whisk in the freezer for about 15-30 minutes. Then place the cold heavy cream and sugar in the cold bowl and beat until soft peaks form. Then add the cold lemon curd and beat just until stiff peaks form. Spread frosting on each cupcake with a knife or spatula, or if you want to pipe the frosting, use a large plain or decorative tip. These cupcakes are best eaten the day they are made but they can be covered and stored in the refrigerator for a few days. Bring to room temperature before serving.

Makes 12 cupcakes

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