I guess you could just weigh out 15g of a bigger bar, but two squaresofchocolate is demoralising and there's something I love about being able to eat the (tiny) entire bar and throw the wrapper away without reaching for more.
For the average cat, weighing 11 pounds or 5 kg, the toxic amount of milk chocolate is approximately 11 ounces, but 1 - 2 squaresof baking chocolate or high quality dark chocolate has the potential to be fatal.