Preheat oven to 325 F. Have ready 4 ramekins (4 to 6 oz each) and an 8 inch metal baking pan.
In a large saucepan, heat the milk and cream just until simmering.
Meanwhile, in a medium size mixing bowl, whisk together the egg, egg yolks and maple syrup. Gradually whisk the hot milk mixture into the egg mixture to temper, whisking until smooth.
Strain into a second bowl. This step is something you can skip if you don't have a sieve, but it helps catch any tiny particles of congealed cream or egg yolk and makes the custards a little creamier.
Whisk in vanilla paste, maple extract, salt and bourbon.
Pour 4 oz of custard into each ramekin. Set all the ramekins in the pan, then boil some tap water in the microwave or over the stove. Pour the boiling water into the pan so that it comes about ⅔ to the top of the ramekins.
Carefully set pan with water and custards in the preheated oven (bottom rack is recommended, but I've used upper rack as well) and bake until centers of custards are just barely set and register 175F. This should take about 35 to 40 minutes, but use a quick read thermometer to check.
Remove from oven. Using a slotted spatula, carefully lift the custards up out of the water bath and set on a rack to cool to rom temperature. Once cool, chill until very cold (about 2-3 hours). You can cover them right away, or let them chill a little bit and then cover about halfway through. Serve with whipped cream or whipped topping.