Prepare oven 350°F.
Line mini muffin tins with liners, spray liners with cooking spray. Melt butter with chocolate chips in a 4-quart heavy bottomed pot over medium high heat, stirring until smooth.
Whisk flour, cocoa powder and salt together, remove pan from heat and whisk in sugar, add in eggs, incorporating one at a time. Stir in flour mixture until just incorporated and spoon batter into prepared muffin liners, fill to the top.
Bake in the middle of the oven 25-30 minutes or until tester comes out with crumbs attached. Cool for 5 mins and dust with confectioner’s powder.
Muffins can be kept in an airtight container for up to 2 days.