I recommend making the lemon curd the night before you make the cupcakes so that it has time to fully cool.
Create a double boiler by bringing 1-2 inches of water to a simmer in a small saucepan. Place a heatproof glass bowl on top and make sure it's not touching the water.
Add the egg yolks, sugar, lemon juice, lemon zest, and salt into the top of your double boiler. Whisk until completely combined and then continue to whisk as the lemon curd cooks (constant whisking prevents the lemon curd from curdling). Continue to whisk the lemon curd for 10-15 minutes or until it become thick and paler in color (it should be similar to the consistency of hollandaise sauce). If the lemon curd isn’t thickening, turn up the heat and constantly whisk.
Once thickened, remove the glass bowl from the heat. Cut the butter into chunks and whisk into the lemon curd until fully combined and smooth. If desire, pass through fine mesh strainer and then pour into a glass jar. Let cool to room temperature and then cover and store in the fridge for up to a week.
Preheat the oven to 350°F and line a 12 cup muffin pan with cupcake liners.
Whisk the cake flour, baking powder, and salt together in a large bowl.
In a separate bowl, whisk the melted butter, sugar, eggs, lemon zest, and lemon extract together until combined.
Stir in about half of the flour mixture. Then stir in the milk and the rest of the flour mixture and mix until fully combined. The batter will be thin.
Pour about ¼ cup of batter into each cupcake liner (they should be about halfway full). Bake at 350°F for 18-22 minutes or until a cake tester comes out clean.
Remove from pan and let cool on wire rack. Let cupcakes cool completely before filling and frosting.
Use a cupcake corer to remove the center of each cupcake, then fill with 1-2 teaspoons of cooled lemon curd.
Create a double boiler by bringing 1-2 inches of water to a simmer in a small saucepan.
In the bowl of a stand mixer, whisk the sugar and cream of tartar into the egg whites then set the bowl over your double boiler. Do not let the bottom of the egg whites bowl touch the water.
Whisk the whites and sugar constantly until sugar is dissolved and mixture has thinned out (about 3-4 minutes). If using an instant thermometer, it should read about 160°F.
Place the bowl on the stand mixer and fit with the whisk attachment. On medium-high speed, beat the mixture until soft peaks form. Then add in the vanilla extract and beat on high until stiff peaks form and the meringue is no longer warm to the touch, at least 10-15 minutes. If it’s still not reaching stiff peaks, stop the mixer, place the bowl in the fridge for 10 minutes, then return to the mixer and continue beating until stiff peaks form.
Frost cooled cupcakes. I used a Wilton 1M tip for these photos. After frosting, use a culinary blow torch to lightly toast the meringue.
Cupcakes are best eaten the same day but can be stored in the fridge for up to 3 days.Store any leftover lemon curd at room temperature for up to a week or use to make lemon poppy seed macarons!