Line an 8-inch by 8-inch baking pan with foil or parchment paper. Be sure to let the foil hang over the edges so you can use it to lift the fudge out of the pan later.
Place a medium-sized saucepan over medium-high heat. To the saucepan add the butter, evaporated milk, and sugar. Bring to a boil, stirring frequently to help make sure everything melts together.
Cook for 4-5 minutes, or until a candy thermometer reach 235 degrees F (just under the "soft ball" phase). Be sure to stir constantly to prevent burning.
Remove from the heat and stir in the marshmallow fluff and chocolate chips until combined and smooth. The mixture will be thick.
Pour in the vanilla and bourbon. Stir to combine. Be carefully not to stir too quickly or the liquids may splash out of the pan.
Pour the fudge into the prepared baking dish and shake gently so it spreads into an even layer. Sprinkle the pecans all over the top of the fudge, if desired.
Leave the fudge to set up on your countertop for 4 hours to overnight. (see note)
Use the foil to lift and fudge out of the baking dish and then gently pull the foil away from the fudge and discard it. Cut the fudge into 36 pieces (6x6 grid). Store in an airtight container in the fridge or on your counter if not serving immediately.
Notes
Nutritional information and yield will vary depending on how large you cut the fudge squares.
You can speed up the resting time by letting the fudge set up in the fridge. Wait about 30 minutes once it's in the pan, and then chill the fudge. It should be firm in about 2 hours.