1. Place half the butter in a medium bowl. Melt the remaining butter in a small saucepan over medium heat.

2. Pour the melted butter into the bowl to combine both butters and allow them both to come to room temperature. It should be creamy looking with small bits of un-melted butter. Set aside.

3. Place flour in a bowl and sift in the cocoa powder. Add the salt and whisk together.

4. Combine the eggs, sugar and the vanilla in the bowl of a stand mixer fitted with a paddle attachment. Mix on medium-low speed. Scrape down sides and bottom of the bowl. With mixer running, alternating between the two, add in the butter and flour in 3 additions. Then mix to combine well.

5. Remove the bowl from the mixer and fold in the chocolate chips. Set aside in a cool spot (not the fridge) for 2 hours. The batter can be refrigerated for up to 2 days but should be returned to room temperature before filling the molds.

6. Preheat oven to 350°F convection or standard.

7. Transfer batter to a pastry bag and pipe evenly into molds. Stop just below the rim.

8. Bake 12 minutes in a convection oven or 16 minutes in a standard oven. Test the bouchons with cake tester until it comes out clean, avoiding the melted chocolate chips.

9. Remove from oven and rest 10 minutes. Unmold and cool completely. Dust the tops with powdered sugar.