Preheat the oven to 350F and butter an 8×8-inch pan or line it with parchment paper with extra hanging over the sides - this makes for easy removal and slicing. (This was Gwendolyn's idea - I usually eschew this step, but I'm glad I took the extra minute. So easy.)
In a medium bowl, stir together the butter and sugar, then add the egg, bourbon and vanilla, stirring until smooth. Add the flour and salt and stir until almost combined; add the chocolate and pecans and stir just until blended. Spread into the pan.
Bake for 25 minutes, or until the edges start to pull away from the sides and the middle is set. Cool at least slightly before cutting into squares.