Preheat oven to 325°F.
Spray a 6-well silicone donut pan with cooking spray. Place the donut pan onto a baking sheet and set aside.
Add butter to a heatproof, medium bowl and melt the butter in the microwave. Immediately add the chopped chocolate, cocoa powder, and vanilla to the melted butter and stir until the chocolate is completely melted and the mixture is smooth and shiny. Set the chocolate mixture aside to cool slightly while you move on to the next step.
In a small bowl, whisk together flour, cornstarch, and salt. Set aside.
In a the bowl of a stand mixer using the whisk attachment (or in a large bowl using a handheld electric mixer), beat together eggs and sugar on medium-high speed until the mixture is light, fluffy, and pale yellow, about 2 minutes.
With the mixer running on medium speed, slowly pour the slightly cooled chocolate mixture into the egg mixture. Continue to beat the mixture on medium speed until well combined, about 30 seconds.
Remove the bowl from the stand mixer (if using) and add the flour mixture. Using a rubber spatula or wooden spoon, gently fold the flour mixture into the batter. When nearly all of the flour is combined, add the chocolate chips and continue to fold until all flour and chocolate chips are combined, being careful not to over mix. Batter will be thick.
Transfer the batter to a piping bag and cut off the end so there's a large hole (or use a plastic food storage bag with one corner cut off). Pipe the batter into the prepared donut pan so that each well is nearly full.
Keeping the baking sheet under the donut pan, put the pans into the preheated oven and bake the bronuts for about 24-28 minutes or until they have risen slightly and a paper-thin crust has formed atop each. The bronuts are finished baking when a toothpick inserted into the center of the pan comes out with just a few moist crumbs and no raw batter.
Place the pan on a wire rack and allow the bronuts to cool completely in the pan. Once completely cooled, remove bronuts from the pan by placing one hand flat under each well and pushing upward rather then peeling one corner of the well away from the bronut, which could break the bronut.
Decorate bronuts as desired and enjoy!