In a small sauce pot, mash blackberries with a fork and then add sugar and bring to a simmer. In a small ramekin, whisk together 1 T cornstarch with 1 T water. Stir into the simmering blackberries and cook on low for approximately 3-5 minutes or until mixture has thickened. Spread out on parchment paper or a non-stick baking mat and let cool completely .
Once cooled, drop medium sized dollops of jam (a little smaller than a golf ball) onto wax or parchment paper sprayed with nonstick spray. Put in freezer until slightly hardened so you are able to pick up and it doesn’t spread out. Once hardened, form into balls.
Next, stir together dry ingredients.
In a mixer, beat together the butter and sugar until fluffy. Add eggs and peanut butter and mix until well combined. Turn the mixer on low and add small amounts of the flour mixture until just combined and no flour is visible. Try not to over mix.
Take a chilled jam ball and form cookie dough around it, making sure there are no cracks for jam to seep through when baked. You want to make sure you don’t press too hard on the dough in order to keep the cookies light and fluffy. The cookies will be large! All of the dough is only meant to make about 10 cookies.
Place formed cookies on a baking sheet lined with parchment paper or a nonstick baking sheet. Using a fork, gently press an indention vertically and horizontally. Bake for 18-20 minutes or until edges just start to brown. You would rather under bake the cookies than overbake. Allow to cool slightly before serving.