Set oven rack in middle position and preheat oven to 350 degrees. Line two 12-cup muffin tins with cupcake liners or spray with cooking spray, and set aside.
Pour milk, egg whites, lemon juice and zest into 2-cup measuring cup, and mix with fork until blended.
Mix cake flour, sugar, baking powder, and salt in bowl of electric mixer on low speed. Add butter and continue mixing on low speed until mixture resembles moist crumbs, with no powdery streaks remaining.
Add all but 1/2 cup of milk mixture to crumbs and beat at medium speed for 1½ minutes. Add remaining 1/2 cup of milk mixture and mix an additional 30 seconds, scrape down the sides and then mix 20 seconds more.
Divide batter evenly into prepared cups. Bake until toothpick inserted in the center comes out clean, 15-17 minutes (8-11 for miniature).
Cool completely before frosting.
For the Raspberry Buttercream:
In a small saucepan combine raspberries and lemon juice; bring to a simmer over medium heat and cook, stirring constantly and gently smashing the blueberries, until thickened and reduced, about 10 minutes. Remove from heat.
Using a fine-mesh strainer over a small bowl, pour in the raspberry puree and push through the strainer with a spatual, which will give you smooth puree with no seeds. Set aside to cool.
In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter on medium-high speed (scraping down the sides as needed) until completely smooth, about 2 minutes. Add in the raspberry puree and beat until completely combined, about 1 minute (the mixture may look very “wet”, but don’t worry!) Add in the confectioners sugar, one cup at a time, on low speed. Once all of the sugar has been added, add in the lemon juice and salt, then beat on high-speed for a full 2 minutes. Now the frosting will thicken up and look as it should!
Add frosting to piping bag with tip of choice and decorate as desired, then top with fresh raspberries and lemon slices, if desired!
*You can use frozen raspberries – do not thaw if you do this.