Place the yolks in a medium, heatproof bowl. Gradually whisk in the sugar and whisk until the mixture is thick enough to form a thick ribbon when the whisk is lifted, about 2 minutes. Beat in the Muscat and orange zest. Place the bowl over a pot of simmering water. Whisking constantly, cook until the sabayon is warm and doubles in volume, about 8 minutes. Pour into 4 wine glasses. Refrigerate until chilled, about 1 hour. To serve, spoon the berries over each serving, and garnish with a mint sprig.