Preheat the oven to 375 degrees F. Butter and flour a 9-inch by 13-inch baking dish.
Cream the butter, dark brown sugar, and light brown sugar together with an electric mixer on medium speed until creamy and fluffy, about 2 minutes.
Beat in the whisky, vanilla, and 1 teaspoon salt. Cook's Note: The mixture may look curdled at this point but don't worry, it will come back together.
Then beat in the eggs, one at a time, until combined scraping down the bowl with a rubber spatula, if necessary.
Lightly break up the pecan halves by crushing them in your hands or coarsely chopping them and stir into the batter. Stir in the remaining 1 3/4 cups flour until just mixed.
Bake until a toothpick inserted in the center comes out clean, 35 to 40 minutes. Cool completely in the pan on a wire rack. Cut into 12 large bars.
Cook's Note: This brownie pays homage to the original drink that inspired the term butterscotch, that is before it was replaced with artificial flavoring during prohibition. The ingredients make these brownies extra special. The addition of real Scotch whisky, a healthy dose of real butter, and the omission of leavening agents, leaves a great textured blondie with balanced sweetness without the harshness of artificially-flavored butterscotch chips.