Preheat the oven to 350°F.

In a medium bowl, whisk together the flour, cornmeal, sage and baking powder; set aside.

In a large mixing bowl, beat the butter with brown sugar until combined. Add egg yolks, lemon zest, and vanilla, then mix in dry ingredients to form dough.

Shape the dough into ¾-inch balls. Place them 1-inch apart on ungreased cookie sheets. Press your thumb into the center of each ball and fill with about ½ teaspoon of blackberry jam.

Bake for 13 minutes or until bottoms are lightly browned. Cool on the sheet for 1 minute before transferring to a wire rack to cool completely.