Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper.
In a medium bowl, whisk together the all-purpose flour, baking soda, and kosher salt until well combined.
In a large bowl using an electric mixer, cream the softened butter and packed brown sugar together on medium speed for 2-3 minutes, until the mixture becomes light and fluffy.
Beat the room temperature egg and 1 teaspoon of vanilla extract into the butter and sugar mixture until fully incorporated and smooth.
Gradually add the dry flour mixture to the wet ingredients, mixing on low speed just until a soft cookie dough forms with no visible streaks of flour.
Scoop the dough into 1-tablespoon sized balls and place them 2 inches apart on the prepared baking sheets. Use your thumb or the back of a measuring spoon to make a deep indentation in the center of each dough ball.
Bake for 10-12 minutes, or until the edges of the cookies are set and just beginning to turn golden brown. The centers will still look soft.
Immediately after removing the cookies from the oven, gently redefine the indentations with a spoon. Let the cookies cool completely on the baking sheet before filling each center with about 1/2 teaspoon of strawberry jam. Whisk together the powdered sugar, milk, and 1/4 teaspoon vanilla to make a thin glaze, then drizzle it over the cookies and top with colorful sprinkles.
