Line an 8x8 pan with two sheets of parchment that sit crosswise and overhang the sides.
Chop the pralines.
Combine sugar, cream, 2 tbs butter, corn syrup, and salt in a thick-bottomed steel or copper saucepan.
Stirring constantly, cook over medium-low heat until the mixture boils.
Add your candy thermometer.
Reduce heat to low and cook without stirring until the mixture reaches the soft ball stage, 113C/236F.
While mixture is cooking, fill the tub of a sink or a large container with about an inch of cold water.
When the mixture reaches the soft ball stage, 113C/236F, remove the pot from the heat, and shock it by placing it in the shallow cold water bath for 30 seconds.
Remove the pot from the cold water bath and, without stirring, drop the vanilla and the last 1 tbs of butter on top. The butter will melt and pool with the vanilla in the center of the mixture. DO NOT STIR!
After about 45-60 minutes, when the temperature of the mixture reaches 49C/110F, grab a wooden spoon or transfer your mixture to the bowl of a stand mixer. [I find the wooden spoon method actually works BETTER than the mixer!]
Beat the mixture by hand or on low until it thickens significantly, turning from a dark chestnut to a sandy brown colour and losing its lustre. [This takes about 10-20 minutes by hand, and can take as little as 5 minutes or up to 25 minutes in a stand mixer.]
If desired, add chopped pralines and stir to combine.
Use the wooden spoon or a rubberized spatula to transfer the fudge into your lined pan and spread evenly.
Cool completely on the counter, about 30-60 minutes.
Slice into 64 small squares or 36 larger ones.
Store well wrapped in plastic on a counter for a few days. Or, if you find your penuche is too soft, wrap well and refrigerate for 2-24 hours.
Or, wrap in plastic, place in an airtight container or ziplock, and freeze.