Preheat your oven to 350°F. Line 3 cookie sheets with parchment paper and set aside.
In a medium bowl, whisk together flour, salt, baking powder, and baking soda. Set aside.
In a stand mixer fitted with the paddle attachment over medium speed, beat butter, sugar, and cocoa powder until well combined, about 2 minutes, scraping down the bowl as needed. Add eggs one at a time, mixing well after each addition Add the cooled, melted chocolate and mix until just blended. Stir in the vanilla extract.
Add the flour all at once and mix on low speed until just blended. Scrap the sides of the bowl to incorporate the dough and cover with plastic wrap. Place the dough in the refrigerator for about an hour.
Once your dough has chilled, using # 70 cookie scoop or a rounded teaspoon, drop spoonful of cookie dough about 2 inches apart on your cookie sheets.
Bake until the cookies are puffed and the tops are cracked, about 10-11 minutes.
Remove your cookies from the oven – And as soon as your cookies are out, use the rounded side of a ½ teaspoon measure to make a small, deep well in the center of each cookie. Let the cookies sit on the cookie sheet for a few minutes – and then transfer them to a rack to cool completely.
To make the filling: Melt the chocolate and butter in a medium bowl set over a pan with simmering water, stirring until smooth. Remove from the heat, stir in the vanilla bean paste and set aside until cool and slightly thickened.
Assembly: Spoon or pipe the chocolate mixture into the wells of the cooled cookies. Set aside until the chocolate firms up, about 15 minutes or so.
These cookies can be stored in an airtight container for up to 3-4 days.
Makes about 50 cookies.
Adapted from: Fine Cooking