Preheat oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or grease lightly.

1. In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
2. In a large bowl, whisk together eggs, sugar, melted butter, milk (or buttermilk), and vanilla until smooth.
3. Stir dry ingredients into the wet mixture until just combined. Do not overmix.
4. Fold in chopped strawberries and rhubarb. Set aside.
5. In a separate bowl, beat cream cheese, sugar, egg yolk, and vanilla until smooth.
6. Spoon muffin batter into each liner about halfway. Add a spoonful of cream cheese filling, then top with more batter. Swirl gently if desired.
7. Bake for 20–25 minutes, or until muffins are golden and a toothpick comes out clean.
8. Let cool in pan for 5 minutes, then transfer to a wire rack to cool completely.