Position two oven racks in upper and lower third of oven. Preheat oven to 350°F/180°C. Line 4 rimmed half-sheet baking sheet pans with parchment paper; set aside. (Or if you only have two, line those).

Whisk together the flour, baking soda and salt in a bowl to aerate and combine; set aside.

In a large mixing bowl whisk together the brown sugar, peanut butter, eggs, melted butter, liquid sweetener and vanilla until well blended. Stir in the dry mixture very well until just a few floury streaks are left, then add the chocolate and keep stirring until everything is blended well.

Drop in golf-ball sized balls evenly spaces on the prepared pans (I do 8 cookies per pan). Flatten cookies with your palm, to about ½-inch (12 mm) thickness. Bake for about 12 minutes total, rotating pans front to back and from upper to lower rack halfway through baking. The cookies should be set around the edges but definitely still a bit soft in the center (this preserves the chewiness). Cool pans on racks. Cookies can be stored in single layers separated by parchment in airtight containers at room temperature for up to 3 days.