Preheat oven to 350F (180C). Prepare the muffin tins with cupcake liners.
Using a microplane, finely grate only the bright yellow outer skin of the lemon.
Combine sugar with the lemon zest in a bowl. Vigorously rub the mixture between your fingers for a few minutes. This friction releases the citrus oils, which are immediately absorbed by the sugar crystals. This is the best natural lemon flavor for cupcakes.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and lemon sugar for a few minutes.
With the mixer still on, add eggs, one at a time.
Scrape down the sides and the base of the bowl.
Pour warm milk and mix again.
Sift in dry ingredients: flour, baking powder, and salt. Beat again until all is combined. Add lemon juice.
Fill muffin tins using an ice cream scoop. Pour batter into muffin tin, filling 2/3 of the way up. Do not fill more.
Bake cupcakes in the preheated oven for 20 minutes. or until a toothpick inserted into the middle comes out clean.
Remove lemon cupcakes from the oven. Cool for a few minutes in the baking tin, then place on the cooling rack. Cool cupcakes completely before decorating with buttercream.
How to Make Lemon Buttercream
In the mixing bowl of a stand mixer, add butter. Using the paddle attachment, beat butter for about 30 seconds.
Add sifted powder sugar in a few parts and beat again.
Add lemon juice and lemon zest. Scrape the sides and base of the mixing bowl with the spatula. Make sure all ingredients are combined.
Transfer lemon buttercream into a pastry bag and decorate cupcakes.
Video
Notes
Make sure all ingredients are at room temperature.