In a medium saucepan, whisk together cornstarch and sugar until blended. Gradually stir in water, then add fresh raspberries and lemon juice. Cook until it thickens and bubbles, about 5-7 minutes. Remove from heat and let it cool completely.

Combine melted unsalted butter, graham cracker crumbs, and sugar in a mixing bowl until it resembles wet sand. Press the mixture into a 9 x 13 inch baking dish and bake at 350°F (175°C) for about 8-10 minutes until golden. Allow to cool.

In a large mixing bowl, beat softened cream cheese until smooth. Gradually add powdered sugar, milk, and vanilla extract, mixing until smooth. Gently fold in thawed Cool Whip until no streaks remain.

Spread the cream cheese filling over the cooled crust, then spoon the cooled raspberry filling over it to create a swirl effect.

Cover with plastic wrap and refrigerate for at least 6 hours. Let sit at room temperature for 30 minutes before slicing.