Preheat oven to 350 degrees. Line two cupcake tins with parchment liners, or grease very well with butter.
Sift the lemon zest into the sugar. In a large bowl, cream the sugar with the butter until very fluffy, about 2-3 minutes. Whip in the eggs one at a time, then the yogurt and the vanilla until smooth. Add the flour, baking powder, baking soda, and salt, mixing until just combined. With the mixer running, slowly stream in the buttermilk until incorporated.
Fill each cupcake liner about halfway with batter. Dollop a teaspoon of lemon curd in the center of each. Top with remaining batter, leaving each cupcake about 2/3 way full. It's okay if some of the curd shows through. Bake for 25-30 minutes until no longer jiggly and an inserted toothpick emerges with just a few crumbs. Cool 10 minutes in the tins, then move to a wire rack to cool completely. If you're in a hurry, pop the cupcakes in the freezer for 15-20 minutes.
To make the frosting, whip together the cream cheese, butter, powdered sugar, 2 tablespoons of lemon juice, and vanilla until light and fluffy. Add more lemon juice, 1 tablespoon at a time, to desired taste. If the frosting is too thick, whip in 1 tablespoon of milk or water (I like using heavy cream) at a time until desired texture is achieved.
Pipe or spread frosting onto each cupcake. Top with additional lemon sugar, as desired.