Preheat oven to 350 degrees. Lightly grease an 8-cup ovenproof casserole with 2 teaspoons of butter.
In a large bowl whisk together the half and half, cream, eggs and orange zest. Add all of the granulated sugar, 4 tablespoons of the brown sugar, the ginger, cinnamon, nutmeg, vanilla and grand Marnier. Whisk until well blended. Add the pumpkin puree and whisk until incorporated. Fold in the bread and walnuts and mix.
Pour the mixture into the casserole, dot the top with the remaining 1 tablespoon butter, and sprinkle with the remaining 2 tablespoons brown sugar. Place the casserole in a water bath and bake until golden brown and puffy; about 1 hour and 15 minutes. About 15 minutes before the pudding is done, make the sauce. Serve the pudding warm with 2 tablespoons of the sauce and top with a mint sprig.
For Kentucky Bourbon Sauce:
Heat the heavy cream with the half and half, vanilla and granulated sugar in a saucepan over high heat; whisking for 3 minutes.
Dissolve 1 tablespoon of cornstarch in the bourbon.
As the cream begins to bubble around the edges, add the cornstarch mixture. Remove from heat and whisk well. Return to heat to low; simmer for 1 minute. Serve immediately or store in refrigerator for up to 24 hours.
Reheat in a double boiler when ready to use.