To make the Chocolate Ganache Filling – In a medium heatproof bowl, place chopped bittersweet and milk chocolate. Set aside. In a small saucepan over medium heat, bring the heavy cream and butter to a low boil. Remove from the heat and pour mixture over the chocolate. Let stand for 1 minute. Using a rubber spatula, stir mixture until melted and smooth (if not completely melted, heat in the microwave for several seconds and stir, until melted). Pour mixture over chilled Ragi Cookie crust and refrigerate until set, at least 4 hours or overnight. You can make the tart a day ahead of time.