Line a 8x8 or 9x9 pan with parchment paper so the bottom and sides are covered. Set it aside.
Add the Sugar, Brown Sugar, Milk and Corn Syrup to a large heavy duty stock pot over medium. Stir it until the sugars are dissolved and the fudge starts to boil. 3 cups Granulated Sugar, 2 cups Brown Sugar, 1 1/3 cups Whole Milk , 2 TBS Light Corn Syrup
Add the candy thermometer to the side of your pot, but make sure it isn't touching the bottom of the pot.
Let the fudge continue to cook, without stirring the whole time. Use a silicone pastry brush dipped lightly in a bowl of water to brush down the foamy residue from the sides of the pot.
Let the fudge cook until it reaches 236 to 238 F. Once it hits that temperature remove the pot from the heat.
Slice the butter into pieces and place them on top of the hot fudge, pour the vanilla and salt on top. But do not stir yet. 6 TBS Unsalted Butter, 2 tsp Vanilla Extract, 1/2 tsp Salt
Let the fudge cool without stirring the whole time, until it drops to 110 F.
Beat the fudge with an electric hand mixer over medium high speed until the butter is totally mixed in. Add in the chopped Pecans (reserving 1 TBS for later). 1 1/2 cups Chopped Pecans
Continue mixing with the hand mixer at the same speed until the fudge starts to lose its shine, but is not totally flat in appearance, and can still flow off the beaters.
Use a rubber spatula to spoon the fudge into the prepared pan, letting it spread as much as it can on its own. You can use the spatula a little as needed to spread it into the corners.
Sprinkle the top of the fudge with 1 TBS finely chopped pecans, and a pinch of flakey sea salt, if desired. 1/2 tsp Flakey Sea Salt
Let the pan sit at room temperature for a few hours until firm enough to slice. The top will have a crust, but the centers will be a little more creamy.