Preheat the oven to 350°F. Spread the walnuts on a baking sheet; place in the oven and bake until they start to smell toasty, about 10 minutes. Keep a close eye (or nose) on them. Pour the nuts onto a clean kitchen towel and use the towel to rub off the skins.

Line an 8-inch square baking pan with aluminum foil and grease with butter or nonstick cooking spray. Spread the walnuts evenly over the bottom. Fill a large bowl with ice water.

Place the 2 tablespoons butter, the sugar, milk, half-and-half, corn syrup, maple syrup, salt, and bourbon in a heavy-bottomed 4- to 6-quart saucepan with a candy thermometer attached and set over medium heat. Stir the mixture with a wooden spoon until it comes to a boil, controlling the heat as necessary to prevent it from boiling over, then allow it to cook without stirring, which can cause crystallization, until it reaches 238°F, 15 to 20 minutes.

Quickly immerse the bottom of the pot in the ice water and let it sit for 5 seconds. Place the pot on the counter and allow the fudge to cool, undisturbed, until the temperature on the thermometer drops to 115°F, 11⁄2 to 2 hours.

Using an electric mixer, beat the fudge until it starts to lose its gloss and thickens, about 20 minutes. Quickly spoon the fudge into the prepared pan, spreading it as best you can with a wooden spoon or rubber spatula. Do not scrape down any of the fudge that clings to the saucepan—this will help prevent unwanted crystallization in the fudge. Allow the fudge to sit at room temperature to cool and set, at least 4 hours and up to 8 hours.

When ready to serve, invert the fudge onto a cutting board and cut into 11⁄2-inch squares. Store in an airtight container at room temperature for up to 1 week or in the refrigerator for up to 2 weeks.