They look like the seed leaves (cotyledons) of miner's lettuce, Claytonia perfoliata. As you might guess from the common name, quite edible but a bit of a pest in the garden. However it is extremely easy to remove just by raking.

That was my first thought too. Can you show the whole plant just to make sure?

Thanks folks.

Here are some more pictures:

New photos just confirm my first impression - Claytonia perfoliata,

Yep. Quite tasty.