Preheat your oven to 350°F (180°C) and prepare a muffin tin with festive cupcake liners. In a mixing bowl, whisk together the all-purpose flour, baking powder, and salt until evenly combined.

In a separate bowl, beat the softened butter and granulated sugar together for about 3-4 minutes until the mixture becomes fluffy and pale. Add in the eggs one at a time, mixing thoroughly after each addition, then pour in the vanilla extract.

Slowly alternate the addition of the dry flour mixture and the milk to the creamed butter mixture. Start and end with the dry ingredients, mixing until just incorporated. Gently fold in the sour cream.

Spoon the batter into the lined muffin tin, filling each cup about three-quarters full. Bake in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center comes out clean.

Meanwhile, prepare the lemon curd. In a double boiler, whisk together the egg yolks, whole eggs, sugar, fresh lemon juice, and lemon zest. Cook over medium-low heat for about 8-10 minutes until it thickens.

In another mixing bowl, beat the softened butter until creamy. Gradually add the powdered sugar, alternating with heavy cream and vanilla extract until the mixture is light, fluffy, and spreadable.

Once the cupcakes are completely cool, create a small hole in the center of each cupcake using a small knife or a cupcake corer. Fill each hole with a generous spoonful of chilled lemon curd, then pipe the fluffy frosting on top.