Combine butter, brown sugar, corn syrup, salt, vanilla, and bourbon whiskey in a saucepan over medium-high heat. Bring to a simmer, then whisk mixture and boil for 1 minute. Remove from the heat and let cool for 5 minutes.
Add eggs and whisk quickly to combine. Stir in pecans. Set aside until mixture thickens a bit and cools down, about 5 minutes.
Preheat the oven to 375 degrees F (190 degrees C). Use the butter to generously grease a 12-cup muffin tin.
Cut each croissant in half, then turn the halves and slice open like you're making a sandwich. Place each piece into a prepared muffin cup and press it down and into the sides to fit and line the cup. Remove any pieces that stick up over the edge of the muffin cup and patch those in where needed to cover the sides.
Spoon the pecan filling evenly into the muffin cups, stirring between each spoonful to ensure you get an even amount of nuts and sauce. Use your fingers to press pecans into the cups and even out the tops so they don't burn. Clean up any syrup that dripped onto the pan.
Bake in the center of the preheated oven until golden brown, about 25 minutes, with a pan on the rack underneath to catch any drips.
Remove from the pan and let sit in the pan for 5 minutes. Turn each pie a bit, then lift out and place on a plate to continue cooling. Let cool completely before serving.