Step 1: Swirl prep
In a small saucepan, combine raspberries, 1 tbsp sugar, and 1 tsp lemon juice. Simmer over medium heat for 5–8 minutes until thickened and jammy. Mash the berries as they cook. Once done, press through a fine mesh sieve if you want a seedless swirl (or leave the seeds if you’re cool with texture). Let it cool while you make the batter.

Step 2: Cupcake batter
Preheat your oven to 350°F (175°C) and line a 12-cup muffin pan with paper liners. In a medium bowl, whisk together flour, baking powder, baking soda, and salt. In a large bowl, beat butter and sugar until light and fluffy—about 2 minutes. Add eggs one at a time, then mix in lemon zest, lemon juice, and vanilla. Beat in sour cream and milk. Add dry ingredients and mix just until combined.

Step 3: Swirl and bake
Scoop the batter into liners, filling each about ¾ full. Drop ½ tsp of raspberry puree onto the top of each cupcake, then use a toothpick or knife tip to swirl it into the batter. Bake for 18–22 minutes, until a toothpick comes out clean. Let cool completely before frosting.

Step 4: Frost that magic
In a large bowl, beat cream cheese and butter until smooth and fluffy. Add powdered sugar, lemon juice, zest, and salt. Beat until silky and pipeable. Transfer to a piping bag (or zip-top bag with the corner snipped) and frost generously.