There is only one way the producers of this butter would tie production to excess solar capacity

The Fischer-Tropsch process can be uses to produce any hydrocarbon product. We don't use enough butter for it to feasibly soak up excess solar generation in the summer.

But we do use enough jet fuel, diesel, and gasoline.

And that's even assuming there's enough excess solar to run the whole thing,

Thats not an assumption. That is the specific problem we need to overcome for solar to replace coal and nuclear. Already, we have summer, daytime generation rates going negative because we have not adequately adapted to the seasonal variation in solar. Those negative rates are massively hurting solar rollouts around the world.

We need massive, seasonal electrical loads to make solar profitable during spring/summer/autumn, so that we have sufficient generation capacity available through winter.

Storage is important for matching the daily generation curve to the daily demand curve, but we can't hope to match seasonal variation. It would be easier to shift power across the equator than to build out enough storage to solve the seasonal variation problem.