Preheat oven to 350 degrees. Line a 12 count muffin tin with paper liners and set aside.
In a medium bowl, whisk together flour, baking powder and salt.
In the bowl of a stand mixer fitted with a paddle attachment, beat butter and sugar on medium high speed until light and fluffy, about 2-3 minutes.
Add eggs, one at a time until well blended. Add vanilla and beat an additional 1 minute.
With a spatula add the flour mixture until just combined, then add the juice, zest and milk being careful not to overmix.
Spoon batter into liners about 3/4 full. Bake for 18-20 minutes or until a toothpick inserted in the middle of the cupcakes comes out clean. Remove from oven and let cool on wire rack.
When cupcakes are cooled, fill with lemon curd. To fill the cupcakes with the curd, use a cupcake corer or pairing knife to remove the center of each of the cupcakes, making a well about one-inch deep. Spoon a small amount of lemon curd into each well, then top with frosting.
While cupcakes are cooling make frosting.
In the bowl of a stand mixer combine butter and sugar. Beat on low speed until blended, about one minute. Add the vanilla and milk and increase speed to medium and continue to beat for 4 minutes until smooth and creamy.
Add more milk, if necessary, until you reach a good spreading consistency.
Frost cupcakes when cooled.