Split the half vanilla bean lengthwise and scrape out the seeds. Put the seeds, pod halves, milk, cream, spices and brown sugar in a heavy-bottomed, medium saucepan. Scald the mixture over low heat. Remove from heat, and let the mixture steep for 1 hour. While the mixture is steeping, reduce the maple syrup by 1/3 over medium-low heat.
Combine the egg yolks and salt in a large bowl and whisk just to blend. Bring cream mixture back to a simmer. Dissolve the reduced syrup into the hot milk mixture.
While gently whisking the yolks, drizzle in the hot cream/syrup mixture so the yolks are gradually warmed. Return the mixture to the saucepan and cook over medium heat stirring constantly with a wooden spoon. Be sure to continuously scrape the spoon across the bottom of the pan so the custard does not scorch. The custard is done when it has thickened slightly and can evenly coat the back of the spoon. Do not let it come to a boil.
Strain the custard through a fine sieve. Press out all the liquid in the vanilla bean and discard bean. Stir in the bourbon and maple extract. Place the bowl with the custard into a large bowl of ice. Let it cool, stirring occasionally.
Transfer mixture to an ice cream machine and freeze according to the manufacturer's instructions. Put the finished ice cream in a storage container and freeze until firm.
Once frozen, mix in walnut brittle and serve.
Combine the sugar and water in a medium saucepan over high heat and cook until it reaches a deep amber color. Meanwhile cover a baking sheet with cooking spray. Remove from heat, stir in the walnuts and pour onto sprayed baking sheet. Let sit until hard and crack into desired pieces.