Beat the softened cream cheese and powdered sugar until completely smooth. Line a small plate with unbleached parchment paper. Drop 1-teaspoon dollops of the cream cheese mixture onto the paper. Add a 1/2-teaspoon dollop of strawberry jam right next to each cream cheese drop.

Place the plate in the freezer for at least 45 minutes until the dollops are rock solid.

In a large bowl, whisk the melted butter, 3/4 cup granulated sugar, and brown sugar until glossy. Whisk in the egg, extra egg yolk, and vanilla extract until lightly colored.

Sprinkle the flour, cornstarch, baking powder, baking soda, and salt over the wet mixture. Gently fold with a spatula just until no flour streaks remain.

Scoop 2 tablespoons of dough, flatten it, and place a frozen cream cheese/jam dollop in the center. Fold the dough over the filling, sealing tightly. Roll in the remaining granulated sugar.

Preheat oven to 350°F (175°C). Place dough balls on a parchment-lined baking sheet. Bake for 12-14 minutes until edges are golden and crinkled.

Let cookies rest on the baking sheet for 10 minutes to stabilize before moving. Serve warm to enjoy the oozing jam center.