Boil water and brown sugar in a small saucepan over low heat until sugar dissolves. Let it boil for 5 minutes. Pour into a bowl and let it cool to room temperature. Blend tamarind pulp and lemon juice into brown sugar syrup. Strain. Transfer to ice cream maker and process according to manufacturer’s instructions. Transfer to an airtight container. Freeze at least 2 hours before serving. Use fresh mint leaves to garnish.