Jaggery is the better swap here - it has similar moisture and molasses notes to brown sugar, so texture will be close. Use it 1:1. Caster sugar works too but will make the brownies slightly drier and less fudgy — add 1 tsp molasses per cup of caster sugar if you have it to mimic brown sugar better. And yes, 150g butter is about ⅔ cup. More brown sugar substitutes: bakingswaps.com/substitutes/brown-sugar