Heat a medium size sauce pan over medium heat melt butter and add brown sugar, stir continuously until sugar dissolves and it begins to simmer and a thick brown caramel begins to form. This usually takes 6-8 minutes for me. (see video below)
When the sauce is thickened and looks like caramel remove from heat and stir in the peanut butter and vanilla extract (if using) until melted.
Spoon out 1/2 cup of the peanut butter fudge mixture and set aside.
Stir in the cocoa powder in the remaining fudge mixture.
Line a loaf pan with wax paper and carefully pour the chocolate fudge into the pan, then top with the peanut butter fudge and swirl.
Freeze 30 minutes or until completely set. Peel the wax paper away from the fudge and slice. I slice the loaf into three long skinny rows, then slice the long rows into 7 chunk size pieces. They measure about 1.3"x1.67" each.
Top with sea salt as desired.
Notes
This made 21 pieces, and each piece has 2.2 net carbs.