Fold a sheet of parchment paper in half. On outside of one half, using a permanent marker, draw a 7-inch square.

In a small microwave-safe bowl, heat chocolate and butter on high in 10-second intervals, stirring between each, until melted and smooth (about 45 seconds total).

In a medium saucepan, whisk together flour, cocoa, and cornstarch. Add sugar, milk, and egg white, whisking until smooth. Stir in chocolate mixture. Cook over medium to medium-low heat, whisking constantly, until mixture forms a very thick, dense, slightly less shiny paste that can be formed into a ball in pan, 10 to 20 minutes. (See Note.) Remove from heat, and let cool for 3 to 4 minutes.

Open parchment, and spoon paste over template; press and spread to form a 7-inch square. Fold parchment to cover paste, and freeze until ready to use.