Melt the chopped chocolate with the cocoa butter in a heatproof bowl over barely simmering water, stirring until fully combined. The mixture should look smooth and glossy, with no thick lumps or dull streaks.

Spoon a little more than half of the melted chocolate into the candy molds and use the back of a small spoon to coat the bottoms and sides. A thin, even shell works best, and freezing it for 10 minutes keeps the filling from blending into the chocolate.

Heat the peanut butter and butter in a microwave-safe bowl in 30 second bursts on high, stirring between each burst until melted. Once you whisk in the powdered sweetener and vanilla, the filling should look thicker, creamier, and more uniform.

Take the chilled chocolate shells out of the freezer and spoon in the peanut butter mixture, stopping just below the top edge. Leave enough room for the final chocolate layer, then return the molds to the freezer for about 10 minutes so the layers stay distinct.

Warm the remaining chocolate gently if needed, then spread it over each filled cup and seal it to the sides. Chill for 20 minutes, or until the tops are completely firm to the touch.