Place the ricotta in a small strainer set over a bowl and refrigerate for 30 minutes to drain the excess liquid.
Transfer the drained ricotta to a food processor or blender, add the brown sugar and orange liqueur and process until smooth. (The ricotta mixture can be made up to 8 hours in advance and covered and kept refrigerated.)
Divide the raspberries among 4 dessert bowls and top each with a 1/4 cup of the ricotta mixture. Add a splash of the orange liqueur or juice, and garnish with mint sprigs.