1Preheat oven to 350 degrees F. Butter & flour a 13 X 9" baking pan.
2Prepare the cake according to package directions, but use 1/2 C of the sherry in place of 1/2 C water. When baked, let cool in the pan for about 10 minutes, then invert onto a wire rack to cool completely. Cut the cake into large cubes, about 1 1/2" wide.
3Place the egg yolks, granulated sugar & remaining 1 C sherry into the top of a double boiler. Whisk until completely combined. Place the top pan over simmering water (do not let water touch the bottom of the top pan). With a wooden spoon, stir over medium heat for about 8 minutes, until the custard is quite thick, about the consistency of mayonnaise. Let cool.
4Whip the cream with the confectioners’ sugar. Take half of the whipped cream & fold into the cooled custard.
In the bottom of a trifle bowl or a 13 X 9" disposable aluminum foil pan, layer 1/3 of the cake cubes, 1/3 of the raspberries & blueberries & 1/3 of the custard. Continue layering, ending with custard. Take the remaining whipped cream & completely cover the top of the custard. Refrigerate at least 4 hours before serving. Serve with additional whipped cream, fresh raspberries, fresh blueberries & a sprig of mint, if desired.