In a saucepan over medium heat, whisk together cornstarch and sugar. Add 1 cup of water, followed by 2 cups of fresh raspberries and a tablespoon of lemon juice. Cook while stirring often for about 5–7 minutes until the mixture thickens and becomes slightly glossy. Once thick, remove from heat and let it cool completely while you prepare the cheesecake components.

In a mixing bowl, combine 1 ½ cups of graham cracker crumbs with ½ cup of melted butter and 2 tablespoons of sugar. Mix the ingredients until well combined, forming a crumbly texture. Firmly press this mixture into the bottom of a 9x13 inch baking dish to create an even layer. Bake in a preheated oven at 350°F (175°C) for about 10 minutes until lightly golden, then set aside to cool.

In a large bowl, beat 2 packages of softened full-fat cream cheese using an electric mixer on medium speed until smooth and creamy, about 2 minutes. Gradually add in 1 cup of powdered sugar, ¼ cup of milk, and 1 teaspoon of vanilla extract, mixing just until combined. Gently fold in 2 cups of Cool Whip until the mixture is smooth and fluffy, ensuring not to overmix.

Spread the cream cheese mixture evenly over the cooled graham cracker crust, smoothing it out with a spatula for an even layer. Carefully spoon the cooled raspberry topping over the cream cheese layer, using the back of a spoon to spread it gently without disturbing the underlying layer.

Cover the assembled cheesecake with plastic wrap and refrigerate for at least 6 hours, or overnight if possible. When ready to serve, slice the cheesecake into squares and garnish with additional fresh raspberries or whipped cream.