In a medium saucepan, melt the nut butter, sugar, and butter over medium heat, stirring constantly, until melted and smooth.
Immediately stir in the chocolate chips until melted (if the mixture is too cool to melt them completely, return the saucepan to low heat to help it along without bringing it to a simmer).
Add the oats, cocoa powder, vanilla, flax, and chopped nuts and stir until well-combined.
Drop by heaping spoonfuls (the medium cookie scoop works great) onto parchment-lined baking sheets. Refrigerate until set.
The cookies will keep in a tupperware-type container (layered between sheets of wax paper or parchment) in the refrigerator for a week or more.
