Preheat the oven to 350ºF (177ºC). Line a 12 count cupcake pan with cupcake liners. Set aside.
In a medium size bowl, toss together the flour, baking powder, and salt. Set aside.
In a large bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar on medium-high speed until light and fluffy (about 3 minutes). Add each egg, one at a time, beating on medium-high speed until fully incorporated, scraping down the sides as necessary. Add the vanilla extract and lemon zest and beat again until combined.
Stir the lemon juice into the milk. Scrape down the sides of the mixer bowl and turn the mixer speed to low. Add the flour in 2 additions, alternating with the milk/lemon juice. Remove bowl from mixer and stir with a spatula until everything is completely combined. Do not overmix this batter.
Spoon batter evenly into prepared cupcake liners (I prefer to use an ice cream scoop with a trigger), being careful not to overfill the liners (I typically aim for filling ⅔ of the way full, using a scant scoop). Bake the cupcakes for 18-20 minutes or until a toothpick inserted in the center of a cupcake comes out clean. Remove from oven and allow to cool completely before filling.
In a medium size bowl with a handheld mixer, or a stand mixer fitted with the paddle attachment, beat the butter on medium speed until creamy, about 2 minutes.
With the mixer on low, add in the powdered sugar, vanilla, and lemon curd. Increase mixer speed to medium and beat until completely smooth, about 3 minutes. Add milk/cream to desired consistency, or you can opt to thin out the frosting with more lemon curd. Add salt to taste.
FILL & ASSEMBLE CUPCAKES
Using a sharp knife, cut a circle into the center top of each cupcake. I aim for about the diameter of a nickel and a depth of about ½". Spoon approximately 1 teaspoon of lemon curd into the hole and replace the piece of cupcake you removed. Push down gently. If some lemon curd spills out, that's ok.
Frost the cupcakes using a large zip top bag with the corner snipped off or a pastry bag fitted with a tip of your choosing. Store cupcakes in the refrigerator up to 5 days. If you need to travel with and serve them, it's ok if they are at room temperature for awhile. Just be sure to refrigerate them to store them.
Room temperature eggs: these incorporate into batter much more easily than cold eggs. Allow to sit at room temperature for 45 minutes. If you don't have time, place eggs in a bowl of warm water for 10 minutes or until ready to use.
Fresh lemon juice: do not use bottled lemon juice for these lemon cupcakes. You will need 2-3 lemons to yield ½ cup of juice.
Milk: I strongly recommend you use whole milk for these cupcakes to impart the best texture with optimal moisture.