Preheat the oven to 350°F (177°C). Line a 12-cup muffin pan with cupcake liners.

Whisk the flour, baking powder, and salt together in a large bowl. Set aside.

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter, sugar, and lemon zest together on medium-high speed in a large bowl until creamed, about 3 minutes. Scrape down the sides and bottom of the bowl as needed. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add eggs and vanilla, and then beat on medium-high speed until combined, about 1 minute. Scrape down the sides and bottom of the bowl as needed.

Add the dry ingredients and then turn the mixer on low speed and slowly pour in the milk and lemon juice. Beat until just combined. Do not overmix.

Pour/spoon the batter evenly into the liners. Bake for 18–21 minutes, or until a toothpick inserted in the center comes out clean. Allow the cupcakes to cool completely before filling and frosting.

Using a sharp knife, cut a circle into the center of the cooled cupcakes to create a little pocket about 1 inch deep. The piece you removed will be sort of cone-shaped. Spoon cooled, thickened lemon curd inside each carved-out cupcake—use however much will fit. (I use a teaspoon measuring spoon for this. Usually you can fit between 1–2 teaspoons of filling in each.) Slice/tear off the pointy end of the “cone” piece of cupcake you removed, and gently press the round piece back on top of the filling. See my How to Fill Cupcakes post if you need extra visuals or help with this step, or if you want to pipe the curd in with a piping tip.

With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2 minutes. Add the confectioners’ sugar, heavy cream/milk, and vanilla. Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 2 full minutes. Taste. Add a pinch of salt if frosting is too sweet. I always add 1/8 teaspoon. Add up to 1/2 cup more confectioners’ sugar if frosting is too thin, or more cream/milk—a Tablespoon at a time—if frosting is too thick.

Frost cooled cupcakes and top each with a lemon slice and/or mint sprig, if desired. You can swipe the frosting on with an icing knife or use a piping tip such as Wilton 1M or Ateco 826.

Store leftover cupcakes in the refrigerator for up to 4 days. A cupcake carrier makes storing and transporting easy.