The plural forms of material nouns such as "butter"
"How many grammes of butter is in the cake?" Is this wrong? The answer says "are in the cake"
If you have a noun (in this case "butter") which follows a preposition, you know that the noun cannot be the subject of the sentence, so the verb does not apply to it.
The previous posters were correct in their comments, and I would like to point out that many non-native speakers of English have a better knowledge of grammar rules in English than we native speakers.
Another native chiming in; the "are" does indeed refer to the grams, not the butter. Butter is (usually?) an uncountable noun, we talk about butter, not about butters! But if it is being measured, then it will be X grams (or ounces or...) of butter, and it is the unit of measurement (grams, spoonfuls, etc) that will govern whether the verb is singular or plural.
Same applies to things like sugar, salt, water... we say X amount OF something, and the unit of measurement is what governs the verb.