Preheat oven to 350F°. Bring milk and butter to room temp. Prepare one cupcake pan with liners. Combine milk and egg whites in a bowl or large glass measuring cup. A measuring cup will be easier to pour from later but a bowl will work too. Add in vanilla and mix well. Egg whites are hard to mix so your end result here won't be perfect but blend as best as you can. Set aside.
In a bowl of a stand mixer, mix flour, sugar, baking powder and salt. Add butter, mixing on low until the entire mixture looks like small moist crumb balls.
Add in half of the milk mixture to the crumbs. Mix on low until combined. Scrape the sides of your bowl well. Add in remaining milk mixture. Continue to mix on low until just blended. Do not over mix.
Using a large cookie scoop, divide batter evenly between cupcake liners. Roughly fill each liner 3/4 full, at the absolute most. Over filling liners will give you less than desirable results. Bake for 15 minutes or until a toothpick or cake tester comes out clean or only has a few moist crumbs clinging on. I wasn't execting my cupcakes to be ready at 15 minutes but that was the perfect time. Any longer and they'd have surely begun to dry out and left me unhappy. So start checking at 15 minutes. If you need to cook longer, check every 2 minutes until you are satisfied.
Remove pan from oven and let cupcakes rest in pan for 5 minutes or so. Carefully trasnfer cupcakes to a wire rack to continue cooling. After 20-30 minutes you can do one of several things. Cupcakes can be popped in the fridge or freezer until ready to frost. You can frost and then pop in the fridge or you can frost and leave out at room temp. I love chilled cupcakes. If you are going to frost and leave your cupcakes out at room temp, make sure the cupcakes have cooled completely.
Frosting: In a medium bowl or stand mixer, cream sugar and butter. Scrape the sides of your bowl and add lemon juice. Mix well, stop to scrap and add just a touch of color if you wish. Wilton icing color is best to use. Do not use liquid food coloring as this will alter the final product. Also, remember that a small amount of color goes a long way. I like to use a toothpick for my coloring purposes. Dip the toothpick just slightly in your icing color then smear it into your icing. Mix well. Transfer to a piping bag or frost as you wish.