Add tofu, cocoa powder, maple syrup, vanilla extract, instant coffee, and salt to a food processor. Blend until mostly combined, then add in melted chocolate. Blend again until smooth. Add plant milk, 1 tbsp at a time, as needed to help blend and achieve desired texture. (Less milk will make a thicker mousse.)
Store leftovers in an airtight container in the fridge. Leftovers will keep for about 3-4 days. The mousse also thickens as it sits in the fridge, so you may need to add extra plant milk before serving.
Notes
1) Use soft/silken tofu for a lighter, fluffier texture. Firm tofu will yield a thicker, pudding-like texture (with more protein).