Preheat your oven to 375°F (190°C). Line a cupcake tin with paper liners to get it ready for the batter. This ensures your cupcakes bake evenly and are easy to remove. Setting this up first will let you move quickly once your batter is ready.

Add the white sugar and grated lemon zest to the bowl of a food processor. Pulse several times until the zest is finely broken up and the sugar takes on a pale yellow color with a fragrant lemon aroma. This step helps distribute bright lemon flavor throughout your cupcakes. I like to take a minute here to really let the sugar get packed with zest.

Sift the all-purpose flour, baking soda, baking powder, and kosher salt into a large mixing bowl. Whisk these dry ingredients together until well combined. Setting up your dry ingredients separately ensures a smoother batter later on.

In a medium bowl, combine the eggs, warmed whole milk, melted unsalted butter, lemon juice, sour cream, and the lemon sugar from Step 2. Whisk until the mixture is smooth and homogeneous; everything should be at room temperature. If the mixture clumps, microwave the bowl for 30 seconds and whisk again to incorporate.

Pour the wet ingredients from Step 4 into the bowl with the dry ingredients from Step 3. Gently mix together just until combined—do not overmix, or your cupcakes might turn out dense. Fill each cupcake liner about 3/4 full with batter. Bake in the preheated oven at 375°F for 5 minutes, then reduce the temperature to 350°F and bake for another 10-12 minutes until the cupcakes are springy to the touch and a toothpick comes out clean.

In a stand mixer or large bowl, cream the unsalted butter, salt, and lemon zest together on medium speed for several minutes until light and fluffy. Sift in the powdered sugar in a few batches, mixing well between each addition to ensure a smooth, creamy frosting. Scrape down the bowl as needed. I always sift the powdered sugar—it makes for the silkiest frosting.

With the mixer on low, add the lemon juice to the frosting from Step 6 and continue to mix until everything is fully combined and the frosting is smooth and spreadable. Scrape down the bowl once more and mix briefly. Transfer the frosting to a piping bag. Once the cupcakes have fully cooled, decorate them as desired with your lemon buttercream. For a zingier finish, I sometimes add an extra sprinkle of grated lemon zest over the top!