1. Chop the chocolate and put into a large heatproof bowl, add butter and set over pot with simmering water. Melt completely. Remove from heat and let cool.
2. In a large bowl or in a bowl of a stand mixer beat eggs and sugar until pale and fluffy. Add vanilla extract, melted chocolate mixture and mix until combined and smooth.
3. Sift in flour, cocoa powder, baking powder and salt. Stir just until incorporated.
4. Refrigerate for 2-3 hours.
5. Preheat oven to 360F (180C).
6. Form the dough into 1-inch balls, roll the balls in the powdered sugar and place on a baking sheet lined with parchment, leaving 2-inches (5cm) apart.
7. Bake for 8-12 minutes. Until the edges are firm and the center still soft.
8. Let cool.