Preheat the oven to 350°F. In a medium bowl, stir together the flour, 4 tablespoons of the butter, and ¼ cup of the brown sugar. Using your fingers, pinch mixture into moist crumbs. Stir in the pecans. Set aside.

In a large bowl, using an electric mixer, beat the remaining 8 tablespoons butter and ¾ cup brown sugar on medium speed until light and fluffy, about 1 minute. Beat in the pumpkin puree, egg yolks, cornstarch, cinnamon, nutmeg, cloves, and salt, then beat in the milk and bourbon.

In a stainless steel bowl, using clean beaters, beat the egg whites until stiff but not dry. Fold egg whites into the pumpkin mixture until no white streaks remain.

Pour the custard into the baked pie shell. Sprinkle the pecan streusel on top. Bake the pie in the preheated oven on the middle oven rack for 1 hour, or until risen and golden and a toothpick inserted in the center comes out with only a few moist crumbs attached. Let cool, and serve.