Carefully pour the bacon fat into a heatproof measuring cup, and add enough melted butter to make 1 1/2 cups. Set aside.
Generously butter a 13-by-18-inch baking sheet and set it aside on a heatproof surface.
Combine the bacon fat mixture with the sugar, vanilla, salt and water in a medium-size saucepan and bring it to a boil over medium-high heat. Insert a candy thermometer and continue to cook, stirring constantly with a whisk until it reaches 300 degrees (hard crack stage), 18 to 20 minutes.
Wearing your oven mitts, remove the pan from the heat and stir in the crumbled bacon. Carefully pour the mixture onto the prepared baking sheet, and use a heatproof spatula to spread it evenly. Cool the buttercrunch completely, at least 1 hour, then break it into bite-size pieces. Store the bacon buttercrunch, layered with parchment or wax paper, in an airtight container at cool room temperature for up to 1 week.