Position rack in lower third of oven. Preheat oven to 375 degrees. Lightly grease a 6-cup popover pan or 6 (6-ounce) custard cups. Set custard cups in a jelly roll pan for easier handling.
2
Sift the flour, granulated sugar, cocoa powder and salt into a medium bowl; set aside. Beat eggs in a large bowl with electric mixer set at a low speed for 1 minute. Beat in milk, melted butter and vanilla. Beat in flour mixture until smooth. Pour batter into the prepared pan or the custard cups. Bake until puffed and firm, about 50 minutes.
3
Place piece of waxed paper under a wire rack to keep the count clean. Remove popovers immediately from pan; place on wire rack. Generously sprinkle powdered sugar over popovers. Serve immediately.