Fold finely diced fresh strawberries into the strawberry preserves. For easier stuffing, drop 1-tsp dollops of this mixture onto a plate and freeze for 30 mins.

Make optional streusel: Mix melted butter, flour, and sugar with a fork until crumbly. Set aside.

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

In a stand mixer, cream softened butter, granulated sugar, and brown sugar on medium-high for 2-3 minutes until fluffy. Add egg and vanilla; beat until smooth.

Gradually add dry ingredients on low speed, mixing just until a soft dough forms. Do not overmix.

Scoop 2 tablespoons of dough, flatten into a disc, and place 1 teaspoon of the strawberry filling in the center. Pinch edges tightly to seal the jam completely, rolling back into a smooth ball.

Place stuffed dough balls on a plate and chill in the refrigerator for at least 45 minutes to prevent spreading.

Preheat oven to 350°F (175°C). Place chilled balls on parchment-lined baking sheets. Flatten tops slightly and press a pinch of streusel on top.

Bake for 12-14 minutes until edges are barely pale golden. The centers will look soft. Let rest on the pan for 10 minutes, then transfer to a wire rack to cool completely.

Whisk powdered sugar, vanilla, and enough milk to make a thick glaze. Drizzle heavily over completely cooled cookies. Let set before serving.