Make the filling: Melt the chocolate with the milk in a heatproof bowl set over a saucepan of simmering water (do not let the bowl touch the water), whisking until smooth. Whisk the egg yolks, sugar and cornstarch in a large bowl; slowly whisk in the chocolate mixture, then pour into a separate saucepan. Cook over medium-low heat, whisking, until bubbles appear around the edges and the mixture thickens, 3 to 4 minutes. Transfer to a bowl and press plastic wrap directly onto the surface. Cool to room temperature, about 45 minutes.
Beat the heavy cream with a mixer on medium speed until soft peaks form. Fold half of the whipped cream into the cooled chocolate mixture using a rubber spatula. Cover and refrigerate until set, about 2 hours; cover and refrigerate the remaining whipped cream until ready to serve.
Make the popovers: Position a rack in the lower third of the oven; preheat to 450 degrees F. Whisk the flour, salt and 1/4 teaspoon cinnamon in a large bowl. Whisk the milk and eggs in another bowl, then whisk into the dry ingredients until smooth. Whisk in the melted butter. Let rest 10 minutes.
Brush a 6-cup popover pan with melted butter; dust with flour and tap out the excess. Divide the batter among the cups. Bake 20 minutes, then reduce the oven temperature to 350 degrees F and continue baking until puffed and golden brown, about 20 more minutes (do not open the oven door). Transfer the pan to a rack; let cool slightly. Meanwhile, mix the sugar and the remaining 1/2 teaspoon cinnamon in a bowl. Remove the popovers from the pan; brush with melted butter and toss in the cinnamon sugar.
Slice the top of the popovers three-quarters of the way through to open, leaving a hinge. Spoon in the filling and reserved whipped cream. Dust with cocoa powder and cinnamon.