Select the layer and copy Ctrl+C or > Edit > Copy, then Select > File > Create > From Clipboard, this creates a new document from the copied layer, or you can just make the layer you want the only thing visible.

Then select > File > Save As... then go down to "Select File Type (By Extension)" just above the help button shown here-

Then start typing png, or scroll down to find "PNG image" , or in the box where it says "All images select that box and go down to "PNG image (*.png)"