Preheat the oven to 350 degrees F and line a 9-by-13-inch baking dish with foil, leaving a 2-inch overhang on all sides; butter the foil. Combine 2 tablespoons butter, 1/4 cup brown sugar and the heavy cream in a small saucepan. Bring to a simmer over medium heat, stirring occasionally; continue to cook, stirring frequently, until slightly thickened, 3 to 4 minutes. Remove from the heat and stir in the scotch and 1 teaspoon vanilla; set aside to cool.
Melt the remaining 2 sticks butter in a large saucepan; let cool slightly. Stir in the remaining 1 1/2 cups brown sugar, the eggs and the remaining 2 teaspoons vanilla, then stir in the flour, baking soda and fine salt.
Spread the batter in the prepared pan and pour the butterscotch sauce on top. Gently swirl the sauce into the batter using a knife. Bake until a toothpick inserted into the center comes out clean, 30 to 35 minutes. Sprinkle with flaky sea salt and let cool completely in the pan. Lift the blondies out of the pan using the foil overhang. Cut into pieces.