**For the Vanilla Cupcakes:**
Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with cupcake liners.
In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
In a large bowl, cream the softened butter and sugar together using an electric mixer on medium speed until light and fluffy (about 3–4 minutes).
Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.
Gradually add the dry ingredients to the butter mixture in three additions, alternating with the milk in two additions. Begin and end with the dry ingredients.
Gently fold in the sour cream or yogurt until fully incorporated.
Divide the batter evenly among the cupcake liners, filling each about 3/4 full.
Bake for 18–22 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cupcakes to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.
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**For the Lemon Curd:**
Set up a double boiler by filling a saucepan with a few inches of water and bringing it to a simmer. Place a heatproof bowl over the saucepan, ensuring the bottom of the bowl does not touch the water.
In the bowl, whisk together the egg yolks, whole eggs, sugar, lemon juice, and lemon zest.
Stir the mixture constantly over medium-low heat for 8–10 minutes, or until it thickens and coats the back of a spoon.
Remove the curd from heat and gradually add the butter, a few pieces at a time, stirring until fully melted and smooth.
Strain the curd through a fine-mesh sieve to remove any lumps. Cover with plastic wrap directly on the surface to prevent a skin from forming, and refrigerate until completely chilled.
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**For the Vanilla Frosting:**
In a large bowl, beat the butter on medium speed until smooth and creamy.
Gradually add the powdered sugar, one cup at a time, mixing well after each addition.
Add 2 tablespoons of heavy cream and the vanilla extract. Beat on high speed for 2–3 minutes. If the frosting is too thick, add an additional tablespoon of cream.
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**Assembly:**
Using a knife or cupcake corer, cut out a small hole in the center of each cooled cupcake.
Spoon or pipe the chilled lemon curd into the center of each cupcake.
Pipe or spread the vanilla frosting on top of each cupcake, covering the lemon curd completely.
Garnish with shredded coconut, a mini Cadbury egg, or an edible flower for a festive finish.