01 - Preheat oven to 350°F. Line 2 baking sheets with parchment paper and set aside.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, and salt until evenly distributed.
03 - In a large bowl, beat the softened unsalted butter, granulated sugar, and light brown sugar using an electric mixer until light and creamy, about 2 to 3 minutes. Add the egg and vanilla extract, mixing until fully incorporated.
04 - Gradually add the dry ingredient mixture to the creamed butter mixture, mixing on low speed just until a soft dough comes together. Avoid overmixing to keep the cookies tender.
05 - Scoop 1 tablespoon of dough and flatten it slightly in your palm. Place 1/2 teaspoon of strawberry jam in the center, then top with another tablespoon of dough. Pinch the edges together to seal completely and gently roll into a smooth ball. Repeat with the remaining dough and jam.
06 - Place the filled dough balls onto the prepared baking sheets, spacing them about 2 inches apart to allow for spreading.
07 - Bake for 12 minutes or until the edges are lightly golden. Allow the cookies to rest on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely before glazing.
08 - In a small bowl, whisk together the powdered sugar, milk, vanilla extract, and a drop of food coloring if desired, until smooth and slightly thick. Spread the glaze over the fully cooled cookies and immediately decorate with rainbow sprinkles. Let the glaze set before serving.