Beat softened butter until creamy, about 1 minute.
Add granulated and brown sugar; beat until light and fluffy, 2-3 minutes.
Beat in egg yolk and vanilla until combined.
Whisk flour, cornstarch, and salt; mix into wet ingredients on low speed until just combined.
Roll dough into 1-tablespoon balls, smooth with no cracks. Roll in sugar if desired.
Press a thumbprint into the center of each ball, pinching closed any cracks.
Freeze shaped cookies 20-30 minutes.
Preheat oven to 375°F. Warm jam until pourable but not hot.
Spoon about 1/2 tsp jam into each indent.
Bake 10-13 minutes, until edges just turn golden.
Cool completely on the baking sheet before serving.
