Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper for easy removal.
In a medium bowl, mix graham cracker crumbs and melted butter until fully combined. Press this mixture evenly into the bottom of your prepared pan to create a firm crust.
Pour the sweetened condensed milk evenly over the crust, making sure every inch is covered.
Sprinkle the white chocolate chips, semi-sweet chocolate chips, toffee bits, and chopped mixed nuts evenly over the condensed milk layer.
Add vanilla extract and a pinch of salt, scattering them over the toppings.
Gently press down with the back of a spatula to help the toppings adhere to the condensed milk layer.
Bake for 25 to 30 minutes, or until the edges are golden brown and the center is set but still a little gooey.
Remove pan from the oven and let cool completely before lifting out and slicing into bars.
Notes
For best results, let the bars cool completely before slicing for neat layers. Store leftovers in an airtight container.