In a mixing bowl, combine the peanut butter and liquid sweetener. Stir until smooth and well incorporated.
Stir in the oats (and a pinch of salt if using unsalted peanut butter). Mix until all the oats are evenly coated.
Roll the mixture into evenly sized balls, adjusting the size based on preference (about 10 for standard cups). Press each ball firmly into the bottom of a silicone muffin tray, packing them down tightly to form a solid base. Alternatively, press the mixture into a small dish or loaf tin if making bars.
In a heatproof bowl, combine the dark chocolate and milk. Microwave in 30-second intervals, stirring in between, until fully melted and smooth. Alternatively, melt the chocolate using a double boiler on the stove for more control.
Spoon the melted chocolate evenly over each oatmeal cup, covering the surface completely.
Place the tray in the freezer for at least 30 minutes, or until the cups are firm and set.
Once set, pop the cups out of the silicone tray. Store in an airtight container in the fridge for up to a week or freeze for up to three months.