I'm going to shamelessly steal from Raymond Chen's comment and show the corrected code:
HWND button = CreateWindowW(L"BUTTON", L"\u27F3",
WS_TABSTOP|WS_VISIBLE|WS_CHILD|BS_PUSHBUTTON, size - 105,
size - 29, 100, 24, hwnd, (HMENU)IDI_BUTTON,
GetModuleHandle(NULL), NULL);
Naturally the font you have selected into the window will need to support the character.