Preheat the oven to 350°F. Line a baking sheet with parchment paper.
Whisk the all-purpose flour and salt in a medium mixing bowl. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, both sugars, and vanilla for about 5 minutes.
Add the egg and mix lightly until incorporated. The mixture may separate slightly but will come together once the dry ingredients are added.
With the mixer on low speed, add half of the dry ingredients and mix until incorporated. Stop the mixer and scrape the sides and bottom of the bowl. Add the remaining dry ingredients and mix just until incorporated.
Scoop about 20-25 g of cookie dough and roll it into a 1-inch ball. Place it on the parchment-lined baking sheet. Repeat with the remaining dough.
Immediately use your thumb or the back of a small round spoon to make an indent in the center of each dough ball. Dip your thumb or spoon in flour as needed to prevent sticking.Note: Space the cookies about 2-3 inches apart on the baking sheet to give them enough room as they bake. Transfer the jam to a pastry bag or small zip-top bag. Cut a small opening in one corner and fill each indent with jam.Note: If you're using a zip-top bag, discard the cut corner immediately to prevent it from getting into the cookies. Bake at 350°F for 15-20 minutes, or until the edges start to turn golden brown.
Add more jam to the centers after baking, if desired, since some of the jam may absorb into the cookies as they bake.
Melt the white chocolate and drizzle it over the cooled cookies. Let it set before serving or storing.