Part 1: Making the Moist Lemon Cupcakes

1. Preheat and Prepare: Preheat your oven to 350°F (175°C). Line a 12-cup standard muffin tin with paper liners.

2. Combine Dry Ingredients: In a medium bowl, whisk together the sifted all-purpose flour, baking powder, and salt. Set aside.

3. Cream Butter, Sugar, and Zest: In the bowl of a stand mixer fitted with a paddle attachment, or in a large bowl with a hand mixer, beat the softened butter, granulated sugar, and lemon zest on medium-high speed for 3-4 minutes. The mixture should become very light, pale, and fluffy. This step incorporates air, which is key for a light cupcake.

4. Add Eggs and Vanilla: Add the room temperature eggs one at a time, beating well after each addition and scraping down the sides of the bowl as needed. Beat in the vanilla extract.

5. Combine Wet Ingredients: In a small bowl or liquid measuring cup, stir together the room temperature buttermilk and the fresh lemon juice.

6. Alternate Wet and Dry: With the mixer on low speed, add the dry ingredients in three additions, alternating with the buttermilk mixture in two additions (begin and end with the dry ingredients). Mix only until just combined after each addition. Be very careful not to overmix the batter.

7. Fill and Bake: Divide the batter evenly among the 12 cupcake liners, filling each one about 2/3 full. Bake for 18-22 minutes, or until a wooden skewer inserted into the center of a cupcake comes out clean.

8. Cool Completely: Let the cupcakes cool in the pan for 5 minutes before transferring them to a wire rack to cool completely. This is a crucial step—you must not fill or frost warm cupcakes.

Part 2: Making the Raspberry Filling

1. Simmer the Berries: In a small saucepan, combine the raspberries, sugar, and lemon juice. Cook over medium heat, stirring and mashing the berries with a spoon, until the mixture starts to bubble and break down (about 5-7 minutes).

2. Thicken the Filling: Whisk in the cornstarch slurry. Continue to simmer for another 1-2 minutes, stirring constantly, until the filling has thickened.

3. Strain and Cool: For a smoother filling, press the mixture through a fine-mesh sieve into a small bowl to remove the seeds. Let the filling cool completely to room temperature.

Part 3: Making the Buttercream and Assembling

1. Make the Raspberry Purée: Place about ¾ cup of fresh raspberries in a blender or food processor and blend until smooth. Press the purée through a fine-mesh sieve to remove the seeds. You will need ¼ cup of this smooth, concentrated purée for the frosting.

2. Core the Cupcakes: Once the cupcakes are completely cool, use an apple corer or a small sharp knife to remove a small core from the center of each cupcake (about ½ inch wide and 1 inch deep). Be careful not to go all the way to the bottom. (You can snack on the little cake cores!)

3. Fill the Cupcakes: Spoon or pipe the cooled raspberry filling into the center of each cupcake.

4. Make the Buttercream: In the clean bowl of your stand mixer, beat the softened butter on medium-high speed for 2-3 minutes until it is very creamy and pale.

5. Add Sugar and Liquids: Reduce the speed to low and gradually add the sifted powdered sugar. Once it’s mostly incorporated, increase the speed back to medium-high and beat for another 2-3 minutes until light and fluffy. Add the ¼ cup of strained raspberry purée, 1 teaspoon of lemon juice, vanilla extract, and a pinch of salt. Beat on low until combined, then increase the speed to high and beat for 2 more minutes until the frosting is smooth, airy, and a beautiful pink color. If the frosting is too thin, add more powdered sugar; if it’s too thick, add a tiny splash of milk or cream.

6. Frost and Garnish: Transfer the buttercream to a piping bag fitted with your favorite tip (like a Wilton 1M for a classic swirl). Pipe a generous swirl of frosting onto each filled cupcake. Garnish with a fresh raspberry and a tiny sliver of lemon zest, if desired.