Preheat your convection (fan) oven to 390 degrees F (200 C). Line a baking sheet with parchment paper.
Whip the butter and sugar together in a stand mixer with a paddle attachment or by hand with a whisk.
Add the vanilla extract and mix.
Combine flour, cornstarch, and baking powder in a separate bowl and whisk/mix together so there aren't any lumps.
Add dry ingredients to the butter/sugar/vanilla mixture and mix together (very low speed with the paddle attachment, or using a spoon/spatula by hand) until no dry spots remain. The dough should come together easily and be sort of a play-doh consistency. If it's feeling sticky, you may want to add another tablespoon of flour.
If the dough is a bit soft at this point, you can put the dough in the fridge for about 10-20 minutes before shaping the cookies. We would also recommend this if it is warm in your kitchen or you are making these in the summertime!
Divide the dough into 12 even balls and spread them out evenly on the baking sheet.
Using your thumb, make an indentation in each ball, flattening out the center of each cookie and making room for the jam.
If you notice that your dough is softening/looking shiny, now is also a good time to pop the cookies in the fridge for 10 or so minutes.
Spoon the jam into the indentations. We aim to fit about 1/2-1 teaspoon of jam in each cookie.
Bake for 10-12 minutes or until very lightly golden on the edges. Allow the cookies to cool on the cookie sheet for 5-10 minutes before moving to a cooling rack to help them set. Enjoy!