Microwave 1/4 cup orange liqueur with 3 tablespoons sugar until the sugar dissolves, about 4 minutes. Cut 1 pound cake horizontally into 4 thin rectangles. Toss 2 cups mixed berries with the orange syrup and 2 tablespoons sugar. Beat 1 cup heavy cream with 2 tablespoons sugar until soft peaks form. Make a 4-layer cake with the whipped cream and berries between each layer. Dust with confectioners' sugar.