Puzzle flower

[vaneramos]

Can anyone identify this flower? I have a good eye and memory for plants, and seldom come across something unfamiliar, but this is. Ironically, we saw them first at the Royal Botanical Gardens on Monday: two exuberant little blossoms in the woods. I tiptoed over with my camera, only to discover the memory card had decided to stay home. Next morning I went into the backyard. Right below our deck, there they were again: two little yellow flowers playing a joke on me. It is nickel-sized and comes with little basal kidney-shaped leaves; reminds me of a buttercup but with twice as many petals. That part of the backyard used to be a small perennial garden, so this is likely non-native.