Hello there! I'll explain this for you, and if you have any more questions, either comment or message me :)
So for this, since you are doubling the portion size, you have to double the amount of materials needed. Since for every two cups you need 3/4 teaspoons of butter.
So you would have to find the change in the portion size (which is easily distinguishable as two) so then you have to add two of the material amounts.
3/4 + 3/4 = 6/4 = 3/2 = 1 1/2 Teaspoons
I hoped this helped! Good luck! :)