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 butter, granulated sugar, and 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 wet ingredients, mixing on low speed just until the dough comes together. Do not overmix.
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 shaped cookies 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.
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 cooled cookies and immediately decorate with rainbow sprinkles. Let the glaze set before serving.