In a small saucepan over medium heat, combine the fresh raspberries and sugar (if using). Stir and cook for about 5 minutes, or until the raspberries break down into a sauce-like consistency.
Push the mixture through a fine-mesh sieve to remove the seeds. Discard the seeds and set the puree aside to cool completely before using in the frosting.
Bake the Cupcakes:
Preheat your oven to 350°F (175°C). Line a cupcake pan with liners.
In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and kosher salt.
Pour in the melted unsalted butter, whole milk, eggs, vanilla extract, and lemon zest. Stir with a rubber spatula until just combined.
Gently fold in the mashed fresh raspberries, being careful not to overmix.
Divide the batter evenly among the cupcake liners. Bake for 18–22 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Transfer to a cooling rack and allow the cupcakes to cool completely before frosting.
Make the Raspberry Buttercream Frosting:
In a medium bowl, beat together the softened cream cheese, unsalted butter, and vanilla extract until smooth.
Gradually mix in the powdered sugar on low speed until fully incorporated.
Add 2 tablespoons of the cooled raspberry puree and mix until smooth. If the frosting is too thick, add a teaspoon of milk at a time until the desired consistency is reached.
Assemble and Serve:
Once the cupcakes have cooled completely, spread or pipe the frosting generously on top.
Decorate with fresh raspberries, a sprinkle of lemon zest, or a small drizzle of raspberry puree for extra flair.
Notes
Mash carefully: Lightly mash the raspberries; over-mashing can make the batter too wet and affect the bake time.
Scrape the sides of the bowl: Use a rubber spatula to ensure all the ingredients are fully incorporated.
Check for doneness: Insert a toothpick into the center of a cupcake, and if it comes out clean, they’re ready.
Chill the frosting: If your frosting becomes too soft while piping, chill it for 10–15 minutes before continuing.