To acquire butter on a knife for the purpose of spreading?

Shave the butter using a butter knife.

From Oxford Languages (via Google)

cut (a thin slice or slices) from the surface of something.
"scrape a large sharp knife across the surface, shaving off rolls of very fine chocolate"

If the requirement for a thin curl is removed, scoop works, and is easily understood, even though a knife is not a a scoop.

I scooped up a generous portion of butter with my knife and spread it over my toast.

An interesting reddit discussion on the related subject of peanut butter

Until relatively recently with the advent of refrigeration, butter was soft, so there's not going to be a word that goes back through the centuries referring specifically and narrowly to the scraping action with the knife that you describe, though the verb skim might do:

He broke into the empty house and went back to the kitchen. He reached into the fridge and took out a tub of butter, setting it on the kitchen counter. He put a slice of bread in the toaster. When the bread popped up, he skimmed a curl of butter from the tub and spread it across the surface, trying not to crush the toast. He wondered how so many Americans could eat the terrible air-filled stuff. A slice of it fresh from the plastic bag could be squeezed down into a ball the size of a shooter marble.

"Scoop up" works for me. Although the knife is not scoop-shaped, you alter the angle of the blade in the soft butter to scoop the butter onto the blade. Specialist butter spreading knives have a curved (but not dished) shape to facilitate the scooping action. From this advertisement:

Butter knives: your breakfast's finishing touch Butter knives are specifically designed to spread butter or other spreads on your sandwich, baguette, toast or crackers. Usually, these knives have a round tip to allow you to scoop up the spread from its container.