In a medium bowl, whisk together the flour, cornmeal, sage and baking powder; set aside.
In a large mixing bowl, use a hand mixer to beat the butter with an electric mixer on medium speed for 30 seconds. Add the brown sugar and beat until combined, scraping the sides of the bowl occasionally. Beat in the egg yolks, lemon zest, and vanilla until combined. Add the dry ingredients and beat until a dough forms.
Shape the dough into 3/4-inch balls. Place the balls 1-inch apart on an ungreased cookie sheet. Lightly press the tip of your thumb into the center of each ball (or use round part of measuring spoon). Fill the centers with about 1/2 teaspoon of the blackberry preserves. Bake 13 minutes or until the bottoms are lightly browned. Cool on a cookie sheet for 1 minute. Transfer to a wire rack; cool completely.
Notes
If you want to use fresh sage, substitute 1 tablespoon of finely chopped fresh sage for the dried.