Double Dark Chocolate Thumbprints

Make the cookies:
1. Position a rack in the center of the oven and heat the oven to 350 degrees. Line 3 cookie sheets with parchment or non stick baking liners.
2. In medium bowl, whisk flour, salt, baking powder and baking soda.
3. In a stand mixer fitted with paddle attachment, beat butter, sugar and coco powder on medium speed until well blended, 5 minutes. Scrape down the beater, add vanilla. Add the eggs 1 at a time, mixing until blended after each addition, about 30 seconds. Continue mixing on medium speed until well blended, 1 minute. Add the cooled melted chocolate, mix until just blended. Add flour mixture mixing on low speed until just blended.
4. Using a 1 tablespoon scoop, drop dough about 1 1/2 inches apart on the prepared cookie sheets. Bake one sheet at a time, until the cookies are puffed and the tops are cracked and look dry. 11-13 minutes.
5. When the cookies are out of the oven, use the rounded side of a 1/2 teaspoon measure to make a small deep well in the center of each cookie. Let cookies cook on cookie sheet for 5 minutes. Transfer to a rack.
Filling:
1. Melt the chocolate and butter in a medium bowl set above a pan of simmering water. Remove from heat, let cool.
2. Spoon the chocolate mixture into the wells of the cooled cookies.
Store in an airtight container for up to 3 days.