Place the cranberries, maple syrup, water and bourbon in a heavy-bottomed saucepan and heat on medium-high heat until the mixture just begins to boil. Reduce the heat to low and allow to simmer for 10 minutes. Remove from heat and allow to cool to room temperature.
Lightly grease the gratin dish with butter and set aside. Preheat the oven to 350 F.
In a small mixing bowl, whisk together the flour, baking soda and salt.
In a large mixing bowl, cream the butter, vanilla and sugar with an electric mixer on medium-high speed. Add one egg at a time to the creamed mixture and whisk until completely combined.
Whisk half of the dry ingredients into the creamed mixture until there are no visible flecks of flour. Repeat with the remaining dry ingredients until completely combined.
With a silicone spatula, fold half of the cranberry-maple syrup sauce into the batter in two circular motions. Do not over-mix the batter. Pour the batter into the prepared gratin dish. Place the dish in a large baking tray and pour enough warm water into the tray to cover the sides of the gratin dish.
Bake in the oven for 30 minutes or until the pudding is firm to the touch. A skewer inserted in the center should come out clean. Remove the gratin dish from the baking pan and allow to cool on a wire rack for at least 10 minutes before serving.
Dust the pudding with confectioner’s sugar. Serve warm with the remaining cranberry-maple syrup sauce on the side.