Put the nut butter and chocolate chips in a 2-cup pyrex measuring cup, and microwave for 60 seconds, stirring until all chips are melted. Microwave another 10 seconds if need be to melt all the chips, but be sure to stir to melt.
When fully melted pour into either the loaf pan (first line it with parchment) OR pour into silicon mold.
At this point, sprinkle top if you want with sprinkles or cacao nibs or more chocolate chips or crushed nuts.
Refrigerate the pan or mold until chocolate is set - probably 30 minutes or more.
You can also speed set in the freezer. You can keep it refrigerated for a week in an air-tight container.