Special equipment: Six 6- to 8-ounce clear glass or plastic wide-mouthed tumblers
For the strawberries: Put the gelatin in a medium mixing bowl. Bring 1/2 cup water to a boil in a small pot, then pour it over the gelatin, whisking until smooth. Add the strawberries and sugar, mixing and coarsely mashing until the most of the strawberries are broken up. Refrigerate until the mixture is cold and slightly thickened, about 1 hour.
For the custard: Combine the milk, sugar, cornstarch and salt in a medium saucepan. Bring to a simmer over medium-high heat, whisking constantly; simmer, whisking, until thick, 2 minutes. Stir in the butter and vanilla, then strain through a fine-mesh sieve into a bowl. Cover the surface directly with plastic wrap or wax paper, and refrigerate until cool.
To assemble: Put a layer of angel food cake cubes in the bottom of six tumblers; be careful not to press down too hard. Divide half of the strawberry mixture among the tumblers. Follow with half of the custard over the strawberry layer. Repeat all three layers.
Beat the cream with the confectioner's sugar until it holds soft peaks. Add the orange extract. Spoon whipped cream on top of the trifles. Garnish with mint sprigs.