Step 1: Calculate the total amount of flour used.
200 g of sugar is used, so the amount of flour used is 300 g (because it follows the ratio 2:3).
Step 2: Calculate the amount of butter used.
Since flour and butter are mixed in the ratio 3:1, for every 3 units of flour, 1 unit of butter is used. So for 300 g of flour, 100 g of butter should be used.
Therefore, the mass of butter used to make the sweet is 100 g.