Step 1.  Line a 9 x 13-inch casserole dish or large rimmed cookie sheet with parchment paper.  Use a medium-sized, light-colored, or metal saucepan, if you have one because they will make it easier to gauge the color of the brittle mixture.  Make space in the fridge for the casserole dish or cookie sheet.

Step 2.  Add butter, sweetener, and vanilla to a medium-sized saucepan and cook over medium heat until fully dissolved and no longer grainy, stirring constantly.

Step 3.  Bring to a low simmer. Lower the heat to low and simmer for 1-3 minutes, stirring constantly, until the mixture reaches a deep brown color.  It’s very easy to burn the caramel, so stop as soon as it starts to become brown.  It will happen quickly.  But don’t take it off the heat if it is still tan in color, because that will make it crumbly and grainy.

Step 4.  Remove from heat, mix in peanuts and pour into the parchment-covered pan. Smooth out the nuts so they are evenly distributed.  Make sure that the brittle mixture is touching all the peanuts to hold the brittle together.

Step 5.  Chill in the refrigerator until set, about 1 hour.

Step 6.  Chop roughly.