Spray a 9x13 baking dish with nonstick cooking spray and line the dish with a sheet of parchment paper, letting it hang over the sides of the dish for easy removal.
Add the butter and sugar to a large, heavy-bottomed saucepan and cook over medium-high heat. Stir constantly, for about 5 minutes, until the butter is melted.
When the butter has completely melted, bring the sugar mixture to a boil. Allow the toffee mixture to boil until a candy thermometer reaches 285 degrees F. Be patient, as this process may take up to 30 minutes.
Remove the pan from the heat and stir in the vanilla extract. Pour mixture into the prepared baking dish, making sure to scrape the sides of the pan to get all of the mixture out.
Allow the candy mixture to sit for 2-3 minutes, then sprinkle the white chocolate. Cover with foil for 5 minutes, then use a spatula to spread the white chocolate over the toffee.