Place mascarpone and prepared whipped topping (or fresh whipped cream) in a large bowl and use a hand or stand mixer to mix until well combined and creamy. Add powdered sugar and vanilla and mix again until combined.
Finely chop 1/2 of a bar of dark chocolate. Set aside.
Spread about 1/4 cup of the mascarpone mixture over the bottom of a 13" x 9" dish. Only use enough to coat the pan. Line the mascarpone coated dish with graham crackers , placing them lengthwise, 3 across the width of the pan. Break a couple of graham crackers in half to fill the space at the end of the dish, cutting the crackers to fit, if necessary. Top the graham crackers with half of the remaining mascarpone mixture. Dot the mascarpone mixture with half the raspberries and sprinkle on half the finely chopped chocolate (approximately 2 tablespoons). Repeat layers, using all of the remaining mascarpone, raspberries and chocolate and end with one last layer of graham crackers.
Heat the heavy cream and corn syrup in a small saucepan over medium heat. Meanwhile, place remaining 1-1/2 bars dark chocolate in the bowl of a food processor and process until finely chopped. When heavy cream is heated and just begins to bubble around the edges of the pan, remove from the heat and carefully pour through the feed tube of your food processor. Pulse the chocolate and cream until the chocolate has completely melted and mixture is smooth and pourable. Pour the warm chocolate ganache over the top of the top layer of graham crackers.
Transfer dish to the refrigerator for at least 1 or 2 hours, or until ganache has set then cover with foil and refrigerate for a total of at least 3 or 4 hours before serving. Can be refrigerated overnight. Best if served within 24 hours and consumed with a couple of days.