Preheat the oven to 375°F. Line a 12-cup cupcake pan with paper liners and set aside.

Add the sugar and lemon zest to the bowl of a food processor, then pulse until the zest is broken up and the sugar is pale yellow in color. Transfer the sugar to a medium mixing bowl. (If you don’t have a food processor, rub the sugar and zest together in a medium bowl with your hands until the sugar is damp and very fragrant.)

Add the butter, eggs, sour cream, milk , and lemon juice to the bowl with the lemon sugar, and whisk together until it is lump-free and well combined.

Sift the flour, baking soda, baking powder and salt into a large bowl then whisk together. Pour the wet ingredients into the flour mixture and fold together just until all of the dry streaks of flour have disappeared.

Fill each cupcake liner almost full with batter (about ¼ cup/60g of batter each).

Bake for 5 minutes, then reduce the temperature to 350°F (do not remove the cupcakes or open the oven door), and bake for another 10 to 12 minutes or until the center of each cupcake is springy to the touch. Let the cupcakes cool in the pan for a few minutes, then remove and place on a wire rack to cool completely.

Combine the butter, lemon zest and salt in the bowl of a stand mixer with the paddle attachment, or in a large mixing bowl using a hand mixer. Beat together on medium-high speed until light and creamy, 2-3 minutes.

Then sift in the powdered sugar a cup at a time, and beat in until well combined between batches. Stop and scrape down the bowl as needed. Then, add 2 tablespoons of lemon juice while mixing on low speed. Scrape down the bowl and mix once more until fluffy, about 1 minute. Add more lemon juice as needed if your frosting feels stiff or if you’d like a zippier flavor.

Transfer the frosting to a piping bag with a decorative tip (I used a large star tip) and pipe it onto the cooled cupcakes.