To prepare a water bath, fill a large bowl about halfway with ice water, then place a large, empty bowl inside of it. Make sure the top bowl is tall enough that it won't sink down beneath the water. Place a sieve nearby.
In a medium bowl, whisk the egg yolks with a fork, then place near your stove. Have a kitchen themometer nearby as well.
Add the milk, cream, and whiskey to a 2 quart saucepan. Whisk in the salt, brown sugar, and flour. Cook on low to medium heat, whisking often, until the liquid simmers. The flour will start to thicken around 150° F (65° C).
When the sauce has thickened a little, turn the heat to low and begin tempering the egg yolks. Pour a serving spoon or medium ladle full of the hot liquid into the egg yolks, whisking constantly until it is completely combined. Add the rest of the liquid one spoon at a time, whisking between each addition.
Pour the sauce back into the pan and return it to the heat. Cook on low, whisking constantly. The sauce should begin to thicken further after reaching about 150° F (65° C.) Cook until the themometer reads 170° F (~80° C).
After the sauce reaches 170° F, remove it from the heat and stir in the remaining 2 tbsp whiskey. Then immediately strain it through the sieve into the bowl above the water bath. Stir occasionally, until it has cooled to about 100° F. You can serve it with the bread pudding immediately, or store it in the refrigerator for about 3 days.
