In a blender, blend the mango chunks with the water until smooth. Transfer to a small saucepan, add the brown sugar, and heat gently over medium heat, stirring just until the sugar fully dissolves. Do not boil or reduce.
Remove the saucepan from heat. Add the red chili flakes, lime zest, lime juice, and the pinch of fine sea salt. Stir and let steep for 4–5 minutes, just until aromatic with gentle heat.
Strain through a fine-mesh sieve into a bowl or measuring jug. Do not press the pulp. Discard solids and let the syrup cool completely.
In a pitcher, combine 80–100 ml of the cooled mango chili–lime syrup with plenty of ice. Pour in the chilled club soda and stir gently once or twice to integrate without killing carbonation.
Pour into ice-filled glasses and garnish with mango cubes, lime wedges, and an optional very light sprinkle of red chili flakes.
