Make the filling: In a small saucepan, whisk jam, 1 tbsp cornstarch, and water. Cook over medium heat, stirring, until thickened, 3-5 mins. Cool completely.

Whisk flour, 1 tsp cornstarch, baking soda, baking powder, and salt in a bowl.

In a separate bowl, cream softened butter and granulated sugar until light and fluffy. Beat in egg and vanilla.

Gradually mix dry ingredients into wet ingredients on low speed. Just before fully combined, add sprinkles and mix until a soft dough forms.

Wrap dough and chill for at least 1 hour.

Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.

Scoop and roll chilled dough into 1.5 tbsp balls. Make a deep indentation, fill with ½-1 tsp cooled jam, and seal with another flattened dough portion. Ensure edges are fully sealed.

Place on baking sheets 2-3 inches apart. Bake for 10-12 minutes until edges are just set. Cool on sheet for 5 mins, then transfer to a wire rack to cool completely.

For the glaze, whisk powdered sugar, 2 tbsp milk, vanilla, and salt. Add more milk as needed. Drizzle over cooled cookies and top with sprinkles.