To make the lemon cupcake batter, preheat oven to 350 degrees F (178 degrees C). Line a cupcake tin and reserve.
In a medium-sized bowl, stir in together the dry ingredients: flour, baking soda, baking powder, and salt. Set aside.
In a small bowl, combine whole milk and both lemon juice and zest. Set aside.
In a large mixing bowl with paddle attachment, beat softened butter for about 1 minute on medium speed. Then add the sugar, beating at low speed until light and fluffy (about 3 minutes). Add eggs, one at a time, to the sugar mixture, beating well on low speed after each addition. Alternately, add the dry ingredients or flour mixture and also the reserved mixture of milk and lemon juice and zest to the egg mixture, beating well after each addition.
Using an ice cream scoop, fill cupcake liners 3/4 full with the batter. Bake for 15 minutes or until cupcakes are puffy and bounce back when lightly touched. Let cool completely on a rack before filling with lemon mousse and meringue frosting.
To make the lemon mousse, whisk all the 3 ingredients together until obtaining a homogeneous mixture. Fill cupcakes with lemon mousse by first cutting out a small plug in the center of each cupcake using a paring knife. The plug should be to about one half of the vertical depth of the cupcake. Reserve the cupcake plug. Pipe lemon mousse into each hole. Close the hole with the reserved plug -- this will require trimming off most of the bottom part of the plug so that when replaced in the hole the plug will be level with the rest of the top of the cupcake, and will not squeeze out the mousse. Place filled cupcakes in the refrigerator.
To prepare the toasted meringue frosting: Place egg whites, sugar, and cream of tartar in the heatproof bowl of an electric mixer. Then, set the bowl over a saucepan containing simmering water. Whisk constantly for about 3-4 minutes or until sugar has dissolved and whites are warm to the touch.
Transfer bowl to electric mixer fitted with the whisk attachment. Beat mixture on low speed and then gradually increase to high until obtaining stiff, glossy peaks (about 5 minutes). Add a few drops of fresh lemon juice, mixing until combined. Using a large star tip or other tip of your preference, pipe meringue onto the cupcakes. Toast meringue with a blow torch and sprinkle lemon zest on top of the toasted meringue. Decorate as desired, serve, and enjoy!