2 cups brown sugar
I like making this in a pan with sharp corners instead of rounded ones like pyrex, just personal pickiness. I also place a greased, heavy duty piece of foil in the pan to prevent sticking. You could use greased parchment paper as well.
Mix milk and cream together and set aside.
Add sugars, syrup, and salt to a heavy bottomed pot, stirring until mixture comes to a boil over medium-high heat.
When boiling hard, add butter chunks slowly. Then, add milk and cream mixture to sugar mixture very slowly (the hot sugar will really boil up at this point so be careful) so that the pan mixture never stops boiling.
Add the vanilla.
Cook, stirring almost continuously (as can burn easily) to firm ball stage (245F) on candy thermometer. It takes about 20-30 minutes of stirring, you should probably pour yourself a glass of wine, and watch the temperature. It seems like it takes forever and then will suddenly creep up on you. Make sure the bottom of the thermometer is in the mixture but not touching the bottom of the pan.
Once at firm ball stage, pour into greased pan and cool for 5 minutes. Then, sprinkle the caramel pan with fleur de sel. Once sprinkled, you can put the pan in the fridge to harden further.
When hardened, you can lift the whole piece of candy out onto a cutting board (thank you, foil), peel off the foil backing and cut into nice rectangles several times. Work quickly. As the caramels comes to room temperature it softens and sticks to the knife.
Wrap the caramels in cut wax paper rectangles and twist ends to close.
Note: We doubled Betsy's original recipe...perhaps you don't need quite as many...if not, just use an 8- by 8-inch square pan instead of the 9- by 13-inch pan that we used and halve the ingredients above.