Step 1
Combine lemon juice eggs sugar and lemon zest for lemon curd in a small saucepan over low heat. Cook whisking constantly until thick enough to coat the back of a spatula or spoon about 10 minutes. Remove from the heat and strain through a sieve into a bowl. Stir in butter until melted; set aside.
Step 2
Preheat the oven to 350 degrees F (175 degrees C). Line a 12-cup muffin tin with paper liners.
Step 3
Sift flour sugar baking powder and salt for cupcakes into a large bowl. Pour milk oil vanilla extract and egg onto the dry mixture. Mix thoroughly with an electric mixer of by hand. Pour in hot water and mix to combine; batter will be thin. Spoon batter into the prepared muffin liners filling about 3/4 full.
Step 4
Bake in the preheated oven until tops spring back when lightly pressed 15 to 18 minutes. Cool in the tin for 5 minutes. Transfer to a wire rack and let cool about 20 minutes.
Step 5
Core each cupcake using either a corer or a knife. Spoon in lemon curd.
Step 6
Pour sugar and egg whites into a large bowl. Place the bowl over a saucepan or pot of simmering water. Whisk constantly until sugar is dissolved and egg whites are warm to between 120 and 140 degrees F (49 to 60 degrees C) on a thermometer 4 to 6 minutes. Remove from the heat.
Step 7
Mix in lemon juice and whip at high speed until stiff peaks form. Pipe meringue onto the cupcakes.
Step 8
Set an oven rack about 10 inches from the heat source and preheat the oven's broiler.
Step 9
Place cupcakes under the preheated broiler until meringue has browned about 2 minutes.