In a large mixing bowl, whisk together the granulated sugar and melted butter until fully combined and glossy.

Add the eggs and vanilla extract. Whisk until the mixture is smooth and well incorporated. While continuing to whisk, slowly stream in the buttermilk and mix until fully combined.

Add the flour, baking powder, and salt to the bowl. Gently fold the dry ingredients into the wet ingredients using a spatula. Mix just until only a few streaks of flour remain. Do not overmix.

In a separate bowl, stir together the chopped strawberries and strawberry jam.

Gently fold the strawberry mixture into the muffin batter, mixing just until distributed throughout. Be careful not to overwork the batter to keep the muffins light and fluffy.

In another bowl, beat the cream cheese, sugar, and egg yolk with an electric mixer on medium-high speed until completely smooth and creamy, about 1-2 minutes. Scrape down the sides of the bowl as needed. Transfer the mixture to a piping bag and set aside.

Rest the batter while you preheat the oven to 425°F. Line every other cup of a 12-cup muffin pan with paper liners to allow space for the muffins to rise. Use a cookie scoop to fill each liner about ¾ full with batter. Insert the tip of the piping bag about 1 inch into the center of each muffin and pipe the cream cheese filling inside, slowly lifting the bag upward as you squeeze so the filling stays in the center.

Bake at 425°F for 5 minutes. Reduce the temperature to 350°F and continue baking for 14-18 minutes, or until the tops are set and a toothpick inserted into the muffin portion (not the cream cheese center) comes out clean.

Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.