In a large mixing bowl, add butter and 3/4 cup sugar. Mix on medium until light and fluffy.

Add in the vanilla and continue mixing on medium.

Add in the flour and mix on low until the flour is almost incorporated. Bump the speed up to medium and let mix until a dough forms a ball.

Place the fridge for about 20 minutes to chill the dough. If you leave the dough in the fridge long enough, it will get super hard and will be impossible to keep together when press your thumb into them.

Preheat oven to 350 degrees. Line a cookie sheet with a silicone mat or parchment paper.

Take a small cookie scoop or two spoons and scoop out dough. Roll into a ball. Place the remaining 1/4 cup granulated sugar in a small shallow bowl. Roll the balls of dough into the sugar.

Place the cookie dough balls on the cookie sheet. Press your thumb into the center of the cookie.

Place about 1/4 tsp. - 1/2 tsp. blackberry jam in the thumbprint you just make in the cookie. Don't overfill the cookies, otherwise you'll have jam all over.

Bake for 13-14 minutes until the cookies are done. The bottom of the cookie will start to turn golden brown.