How to Make It
Prepare the Cupcakes: Preheat oven to 350°F. Line a 12-cup muffin tray with cupcake liners; set aside. Beat sugar and butter with a stand mixer fitted with a paddle attachment on medium speed until creamy, about 1 minute. Add eggs 1 at a time, stopping to scrape down sides of bowl as needed. Beat in vanilla until combined.
Whisk together flour, baking powder, and salt in a separate bowl. Add flour mixture to butter mixture alternately with milk, beginning and ending with flour mixture, beating on low speed until just combined after each addition. Stop to scrape sides of bowl using a rubber spatula. Fold in lemon zest and juice.
Scoop slightly rounded mounds of batter using a 2-inch cookie scoop, and drop into cupcake liners. Bake in preheated oven until a wooden pick inserted into centers of Cupcakes comes out clean, 18 to 20 minutes. Let cool in tray 5 minutes. Transfer to a wire rack; let cool completely, about 30 minutes.
Prepare the Frosting: Mash raspberries in a small bowl using a fork. Transfer mixture to a fine mesh strainer set over a small bowl, and push on berries using back of a spoon to strain juice. Discard solids. (You should have about 1/2 cup raspberry juice.) Set juice aside. Beat powdered sugar and butter with a stand mixer fitted with a paddle attachment on medium speed until smooth and fluffy, about 2 minutes. Add vanilla and 1/4 cup raspberry juice. Beat on low speed until combined, about 30 seconds. Add additional juice 1 tablespoon at a time until desired consistency is reached. Transfer Frosting to a piping bag with the tip cut off (about 1/2 inch).
Pipe Frosting onto Cupcakes. Garnish with lemon slices and raspberries.