Preheat the oven to 350°F (175°C) and line a 12-cup muffin tin with liners.

In a small saucepan, combine raspberries, sugar, and 1 teaspoon lemon juice. Cook over medium heat until the raspberries break down and the mixture becomes saucy, 4–6 minutes. If you want a thicker swirl, whisk in the cornstarch slurry and cook 1 minute more. Strain through a fine mesh sieve to remove seeds (optional). Chill while you make the batter.

In a medium bowl, whisk together flour, baking powder, baking soda, and salt.

In a large bowl, beat the softened butter and granulated sugar until light and fluffy, about 2–3 minutes. Add eggs one at a time, mixing well after each.

Stir in sour cream (or Greek yogurt), milk, lemon zest, lemon juice, and vanilla extract until combined.

Add the dry ingredients in two additions, folding gently until just combined. Don’t overmix.

Spoon batter into liners, filling each about two-thirds full. Drop a teaspoon of the raspberry sauce into each cupcake and gently swirl with a toothpick.

Bake for 18–22 minutes, or until a toothpick comes out with just a few moist crumbs. Cool in the tin for 5 minutes, then transfer to a rack to cool completely.

Beat butter until creamy. Gradually add powdered sugar, lemon juice, and zest. Adjust texture with cream if needed. Taste and add a pinch of salt.

Pipe or spread the lemon buttercream on cooled cupcakes. Optionally, spoon a few dots of reserved raspberry sauce on the frosting and lightly swirl.