Preheat oven to 350°F. Line an 8x8-inch pan with aluminum foil or parchment paper. Spray lightly with cooking spray.
PREPARE THE CRUST:
In a small bowl, mix crumbs and melted butter. Press mixture firmly into the bottom of the prepared pan.
PREPARE THE FILLING:
In a large bowl, use and electric mixer to combine the cream cheese and sugar. Mix in the eggs. Place the milk in a small bowl, and slowly whisk in the cornstarch, whisking until smooth. Add the milk/cornstarch and lemon juice to the bowl and mix again until it is blended and creamy.
Pour the filling mixture on top of the crumb crust. Scatter raspberries and blueberries on top. Bake 35 to 40 minutes, or until touch to center feels slightly firm and set.
Cool completely (I like to refrigerate for a couple hours) before cutting. Use foil or parchment to lift the cheesecake out of the pan and onto a cutting board. Use a thin, sharp knife to cut into clean-looking bars (wiping blade clean with a paper towel between cuts). If you want every single bar to have clean cuts all around, cut off the browned edges of the whole thing before cutting into 16 bars. Refrigerate until ready to serve.