Preheat the oven to 350 degrees F. Line 3 muffin tins with liners (or use fewer muffin tins and work in batches) and set aside.
In a medium-sized bowl, whisk the flour, baking powder, baking soda, and kosher salt together. Set aside.
In the bowl of a stand mixer, fitted with the paddle attachment, cream the butter and sugar on medium speed until it is light and fluffy.
Add the eggs, one at a time, beating to fully incorporate. Add the vanilla extract.
Add half of the flour mixture, and beat well. Add the buttermilk in a slow stream, and beat well. Add the remaining flour mixture and beat until just combined.
Divide the batter evenly between the 36 cups of the muffin tins. Bake for 20-25 minutes, or until the top is golden brown and a toothpick, when inserted into the center, comes out clean. Allow the cupcakes to cool in the muffin tins for 15 minutes before removing them to a wire rack and allowing them to cool completely.
In the bowl of a stand mixer, fitted with the paddle attachment, beat the butter for a couple of minutes, or until soft.
Add 2 cups of powdered sugar, along with the kosher salt and vanilla extract. Beat on low to combine. Add another 2 cups of powdered sugar and beat to combine along with 2 tbs of the milk/cream.
Add the remaining 2 cups of powdered sugar and 2 tbs of milk/cream to achieve the desired consistency of frosting.
Beat on medium speed for a few minutes until the frosting is smooth and fluffy.
Insert a paring knife into the top of a cupcake at a 45 degree angle. Insert the knife only about 2/3 of the way into the cupcake, so that the filling won't leak out of the bottom.
Cut a cone out of the top of the cupcake. Keep the tip of the knife inside the center of the cupcake. Rotate the cupcake while cutting around the top of the cupcake in a circle. Aim to make the hole about 1 inch wide. Pull the cone of cake out of the cupcake.
Cut the cone in half and discard (or eat!) the pointed part. You will use the top, flat part to plug the hole at the end.
Spoon the prepared (or store bought) lemon curd into the holes in the cupcake, filling not quite the whole way. Top with the reserved flat part of the cored out cupcake.
Frost as desired.