How to Make It
In a microwave-safe glass bowl, microwave the chocolate, stopping and stirring every 30 seconds, until melted and smooth. Set aside to cool to room temperature. Combine the flour, salt, baking powder, and cocoa powder in a large mixing bowl.
In a large bowl, beat the butter and sugar with an electric mixer until white and fluffy, about 3 minutes. Beat in the eggs one at a time, then stir in the melted chocolate.
Pour the butter mixture into the flour mixture, folding them together gently until just combined. Cover dough with plastic wrap and refrigerate at least 1 hour or up to 3 days.
Heat the oven to 350° F, and line 3 baking sheets with parchment paper. Roll the cookie dough into tablespoon-sized balls and place them on the prepared baking sheets with 2 inches of space between them. Bake for 10-12 minutes, until the tops are just barely firm. While the cookies are still hot, make a well in the center of each cookie with your thumb.
Transfer to a rack to cool completely. Store cookies between layers of parchment in an airtight container at room temperature until you’re ready to assemble them (or up to 3 days).
Make the ganache: In a microwave-safe glass bowl, microwave the white chocolate, stopping and stirring every 30 seconds, until melted and smooth. In a small pan heat the heavy cream and butter until very gently bubbling. Immediately pour the cream mixture into the chocolate. Add the orange zest and salt, and stir with a rubber spatula until smooth. Allow the ganache to cool completely. Stir in the vanilla.
To assemble, spoon ½ teaspoon of softened ganache into the center of each cookie.
Storage tip: You can refrigerate the filled cookies in a single layer covered in plastic for up to 24 hours. Remove the cookies from the refrigerator 1 hour before serving so they can come up to room temperature.