Preheat the oven to 375°F (190°C) and line a cupcake tin with paper liners.

In the bowl of a food processor, combine the granulated sugar and lemon zest. Pulse several times until the zest is finely broken down and the sugar takes on a pale yellow hue, infused with citrus fragrance.

Sift the all-purpose flour, baking soda, baking powder, and kosher salt into a large mixing bowl. Whisk thoroughly to distribute the leavening agents and salt evenly throughout the flour. Set aside.

In a separate medium-sized bowl, whisk together the eggs, whole milk, melted butter, fresh lemon juice, sour cream, and the lemon-infused sugar. Ensure all ingredients are at room temperature to prevent clumping. If the mixture appears lumpy, microwave it for about 30 seconds and stir until smooth.

Pour the wet ingredients into the dry ingredients and gently mix until just combined. Take care not to overmix to maintain a tender crumb. Fill each cupcake liner about three-quarters full with the batter.

Bake in the preheated oven at 375°F for 5 minutes, then reduce the temperature to 350°F (177°C) and continue baking for an additional 10–12 minutes. The cupcakes are done when they spring back lightly when touched. Allow them to cool completely on a wire rack before frosting.

In a large bowl, cream the softened butter along with the salt and lemon zest for several minutes until the mixture is light and fluffy. Gradually sift in the powdered sugar in batches, mixing thoroughly between each addition to ensure a smooth consistency.

Scrape down the sides of the bowl as needed. With the mixer on low speed, pour in the lemon juice and continue to mix until fully incorporated. Scrape the bowl down once more and give the buttercream a final mix for a silky finish.

Transfer the lemon buttercream into a piping bag fitted with your preferred tip and frost the cooled cupcakes generously, creating beautiful swirls full of bright lemon flavor.