Preheat oven to 350°F (175°C) and put rack in middle of oven.
Line sheet pans with parchment paper. Don't do this without parchment paper. Really.
Whisk together all purpose flour, hazelnut flour (if using), salt, and baking powder together in a bowl.
In a stand mixer cream the butter on medium speed until creamy. Should take about a minute.
Add the 150-grams of sugar and beat until fluffy, maybe another 2 minutes.
Scrape down the sides.
Add egg, yolk, and vanilla and mix on low speed until combined.
Add the flour/salt/soda mixture on low speed until combined.
Scrape off the paddle and scrape down the sides with a spatula, essentially finishing the mixing by hand.
Put 50 grams of baker's sugar into a shallow bowl.
Take the dough in 15-gram (~ 2 teaspoon) increments and roll into a ball. Roll in the sugar and put on a prepared parchment-lined sheet pan. You should get 12 per pan.
Using your thumb or the back of a small, round ½-teaspoon measuring spoon, flatten the balls a bit and create an indentation. Fill each indentation with a heaping ½ teaspoon of your jam.
Bake one pan at a time, 5-6 minutes, turn, then bake another 5-6 minutes until edges are set and bottoms are getting browned.
Remove pan to rack to cool some before removing cookie to a rack to cool completely.