Add dates. Start with about 4 or 5. Begin pulsing together, adding more dates until the mixture is crumbly and sticking together between your fingers. Press into the cups as thick or thin as you'd like, but at least 1/4 inch thick.
Using a high-speed blender or food processor, blend the avocados, cacao powder, date paste, and mint extract together until smooth. Stream in just enough water to help blend if needed. For approximate amounts to start with for a standard muffin pan, try about 3-4 large avocados, 1/2 cup cacao powder, 3-5 dates, and a capful of mint.