In 2 very cold electric mixing bowls, add 2 cups of cream to each. Add 1 cup of melted white chocolate to one bowl and 1 cup dark chocolate to the other. Using an electric mixer, whip both bowls until cream is firm, velvety but still spreadable about 3 to 4 minutes. Line a large loaf pan with parchment paper. Spoon 1/2 of the white chocolate mixture evenly on the bottom of the pan. Spoon 1/2 of the dark chocolate mixture over the white chocolate layer, spread evenly. Top the dark chocolate layer with the remaining white chocolate mixture. Cover the pan completely with plastic wrap and place in the freezer. Freeze until the terrine is well set, about 2 hours. Remove the terrine and unmold. Using a hot knife, slice the terrine into 2-inch slices. Place the remaining chocolate mixture in a pastry bag with a star tip. Spoon a small pool of the sauce in the center of the plate. Lay one piece of the terrine in the center of the sauce. Pipe a small portion of the chocolate mixture off to the side of the terrine. Garnish with the mint, powdered sugar, and chocolate cutouts.