Preheat the oven to 450 degrees. In a stainless steel bowl, whisk all the ingredients together, until frothy. Place the bowl over a double boiler. Whisk the mixture for 2 minutes and remove from the heat. Continue whisking for 20 seconds. Fold in the raspberries. Place 2 slices of the pound cake in each individual gratin dishes. Spoon a fourth of the sabayon over each dish, covering the cake completely. Place the gratin in the oven and bake until the top is golden brown, about 3 minutes. Remove from the oven and place on a plate. Garnish with mint.