Line a 8×8-inch baking pan with parchment, leaving 2-inches overhang on two opposing sides. Make sure to fold the paper into the corners of the pan.
In a heavy saucepan over medium-low flame, heat brown sugar, white sugar, milk, butter, and salt to 236°F (soft ball stage) while stirring constantly. This should take 20-25 minutes.
Pour penuche into the bowl of a stand mixer. Beat on medium for 2-4 minutes, or until fudge is thick and smooth. Lower speed to low and add vanilla extract and chopped nuts.
Using a spatula greased with butter, spread penuche fudge into lined pan, smoothing out the top.
Refrigerate for 30 minutes before cutting. If refrigerated, penuche will keep in a sealed container for up to a week. For maximum freshness, don’t cut until ready to serve