Heat the oven 180°C. Mix flour, sweetener and cocoa powder in a small bowl until thoroughly combined. Add wet ingredients: melted butter, egg, coffee or water, and vanilla. Stir with a rubber spatula to combine. Do not overmix. Gently fold in cherries.
Pour the batter into a prepared ramekin or small dish. Top with chocolate chips or walnuts (or more cherries). Bake for 15 minutes for a gooey centre or 20 minutes for a cakey brownie. Cool the brownie: Remove from the oven and cool for 5 minutes before enjoying. Serve with ice cream, if desired.