Preheat the oven to 350ºF (180ºC). Lightly brush a 6-cavity donut tin with melted butter and set aside. Fit a large piping bag with a large round tip (I use the Wilton 1A round decorating tip). Set aside.
Add the milk, melted butter, vanilla, egg, and sugar to a large mixing bowl. Whisk until smooth and well combined.
Sift the flour and cocoa powder into the bowl. Add the baking powder and salt. Use a rubber spatula or wooden spoon to fold the batter together until the flour is just mixed in.
Fill the piping bag with all of the chocolate batter. Twist the top of the bag to prevent batter from leaking out. Gently but firmly pipe one smooth thick circle of donut batter into each donut cavity in the tin. The batter is very thick! Six thick piped donuts should use all of the batter. If you have batter leftover in the piping bag, add a bit extra to each donut until the batter is all gone.
Lightly wet a fingertip and smooth any areas where the batter starts and ends so the top of each donut looks smooth.
Place the tin in the oven and bake the donuts for 8 to 10 minutes, or until the tops spring back when gently pressed with a fingertip. Transfer the tin to a wire cooling rack. Cool the donuts in the tin for 10 minutes, then transfer them to the rack to cool completely before decorating.
To Glaze
Place the melting wafers in a microwave-safe bowl. Heat the chocolate on full power for 30 seconds. Stir the chocolate. Repeat until the chocolate is completely smooth when stirred. Use a small spoon to “pour” chocolate over the top of each donut. Decorate the with sprinkles or anything else you like, then let the donut sit until the chocolate sets, at least 30 minutes, before serving.