1. Preheat oven to 350ºF (177ºC) and line a 12-count cupcake tray.

2. Whisk together flour, baking powder, baking soda, and salt in a bowl.

3. In another bowl, beat melted butter, sugar, egg, yogurt, milk, and vanilla until smooth.

4. Slowly mix dry ingredients into the wet ingredients just until combined. Fold gently to remove lumps.

5. Remove ½ cup of the vanilla batter, mix it with cocoa powder and milk to make the chocolate batter.

6. Layer batters in liners: 1 tbsp vanilla, 1 tsp chocolate, repeat. Fill only ⅔ to ¾ full.

7. Swirl batters with a toothpick using gentle figure-eight motion.

8. Bake for 15–17 minutes until a toothpick comes out mostly clean. Cool completely.

9. To make buttercream, whisk egg whites and sugar over simmering water until dissolved and 160ºF.

10. Whip to stiff peaks, then beat in butter one tablespoon at a time. Add vanilla and salt.

11. Split the buttercream. Add cooled chocolate to one half, mix until combined.

12. Pipe swirled frosting using both buttercreams side-by-side in a piping bag fitted with a 1M or 2D tip.