In a mixing bowl, mix together the butter and peanut butter until creamy.
Add brown sugar and continue to mix until well incorporated.
Add eggs, one at a time and vanilla extract and continue to mix until everything is well incpororated. Use a spatula to make sure there is no butter on the bottom.
In a separate bowl, sift together the flour, salt and baking powder. With the mixer on low, slowly add the flour mixture until just mixed through. Use a spatula to mix any leftover butter/peanut butter.
Wrap dough in plastic wrap and refrigerate for at least 20 minutes
When dough is ready, use a melon-baller or cookie scooper to scoop out about a tablespoon of dough. Roll dough into a ball and use your thumb to make a "thumbprint" in the middle of the cookie. Roll sides of cookie into chopped nuts and place on non-stick baking sheet. Continue with the rest of the dough.
When all cookies are on baking sheet, use a small teaspoon to add fruit jam into the thumbprint.
Bake at 350 degrees F for 15 minutes, or until the outsides of the cookie begin to brown.
Allow to cool on a wire rack.