Set the oven to 350°F (175°C) and prep your cupcake pan with liners to keep things neat and simple. 

Combine cake flour, baking powder, baking soda, salt, granulated sugar, light brown sugar, and lemon zest in a large bowl (or stand mixer bowl). 

Stir until everything is nicely mixed, the lemon zest will instantly brighten the whole bowl with its fresh, citrusy aroma.

Add the room temperature butter to the dry ingredients and beat on low at first, then increase to medium. The mixture will look a bit sandy or like fine crumbs, which is exactly what you want, it helps create that soft, tender cupcake texture.

Whisk together the eggs, sour cream, oil, and vanilla in a separate bowl until the mixture is smooth and well incorporated. 

Slowly pour this mixture into the dry ingredients and mix until just combined. Don’t worry about overmixing, it's okay if there are a few tiny lumps.

With the mixer on low, slowly stream in the milk until fully incorporated. The batter will look thinner than usual, and that’s perfectly normal, this is what gives the cupcakes their extra-soft, fluffy crumb.

Scoop the batter into the cupcake liners, filling them just a bit over halfway full. 

Bake for 13 to 15 minutes but start checking around the 13-minute mark. Check for doneness by inserting a toothpick, if it comes out clean, they’re good to go. They should look pale, not golden brown.

Once baked, carefully remove the cupcakes from the pan and let them cool completely on a rack or clean surface.

Start the frosting by whipping room-temp butter on high for 3 minutes, until it's soft, smooth, and cloud-like. 

Gradually add the powdered sugar, mixing slowly at first to avoid a sugar storm, then increase speed until everything’s smooth.

Add in the lemon zest and a pinch of salt, then beat again until combined. You can adjust the texture by adding more powdered sugar if needed.

Once your cupcakes are fully cooled, frost them however you like, use a piping bag or just swirl it on with a spoon. Either way, they’re bright, soft, and bursting with lemony joy.