Preheat the oven to 350°F (177°C). Line a 12-cup muffin pan with cupcake liners.

Whisk the flour, baking powder, and salt together in a large bowl. Set aside.

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter, sugar, and lemon zest together on medium-high speed in a large bowl until creamed, about 3 minutes. Scrape down the sides and bottom of the bowl as needed. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add eggs and vanilla extract, and then beat on medium-high speed until combined, about 1 minute. Scrape down the sides and bottom of the bowl as needed. Add the dry ingredients and then turn the mixer on low speed and slowly pour in the milk and lemon juice. Beat until just combined. Do not overmix.

Pour/spoon the batter evenly into the liners. Bake for 18–21 minutes, or until a toothpick inserted in the center comes out clean. Allow the cupcakes to cool completely before filling and frosting.

Using a sharp knife, cut a circle into the center of the cooled cupcakes to create a little pocket about 1 inch deep. The piece you removed will be sort of cone-shaped. Spoon jam inside each carved-out cupcake—use however much will fit. (I use a teaspoon for this. Usually you can fit between 1–2 teaspoons of filling in each.) Slice/tear off the pointy end of the “cone” piece of cupcake you removed, and gently press the round piece back on top of the filling. See my How to Fill Cupcakes post if you need extra visuals or help with this step.

Beat softened butter on medium speed for about 3-4 minutes until completely smooth and creamy. Add the confectioners’ sugar, jam, milk, vanilla extract, lemon juice, and salt. Start the mixer on low speed and beat for 30 seconds. Turn it up to high speed and beat for 1 minute or until combined. Beat in more confectioners’ sugar if frosting is too thin, is separating (due to the juicy jam), or looks too wet. Taste; beat in more salt if too sweet.

Frost cooled cupcakes and top each with a fresh raspberry and lemon slice, if desired. You can swipe the frosting on with an icing knife or use a piping tip such as Wilton 1M or Ateco 826. This is a softer-style buttercream and does not crust. If the cupcakes sit out for several hours, or are in the direct heat, the frosting will begin to lose shape. I recommend refrigerating the cupcakes for at least 1 hour before serving, to help set the frosting.

Store leftover cupcakes in the refrigerator for up to 4 days. A cupcake carrier makes storing and transporting easy.