In a bowl whisk together the yolks until they are combined, add the granulated sugar, a little at a time, beating, and beat the mixture until it is light and lemon colored. Add the cornstarch and the flour, a little at a time, beating, and beat the mixture until it is combined well. Add the milk, scalded, in a stream, beating, and beat the mixture until it is combined well. In a heavy saucepan bring the mixture to a boil, stirring, add the zest, and simmer the mixture, stirring, for 3 minutes. (The mixture will be thick and custardlike.) Remove the pan from the heat and beat in the vanilla. Strain the pastry cream through a fine sieve into a bowl and chill it, its surface covered with a buttered round of wax paper, for 1 hour. Arrange the grapes in the bottom of a 6-cup flameproof gratin dish, spoon the pastry cream over them, spreading it evenly, and sprinkle the almonds over the cream. Sprinkle the brown sugar over the almonds and broil the gratin under a preheated broiler about 4 inches from the heat for 3 to 4 minutes, or until it is bubbly and golden on top.