Without auto-clickers: if your keyboard has a numeric keypad (a lot of new laptops don't)
Alt+shift+numlock
Turns on MouseKeys (your numeric keypad becomes a mouse simulator).
Put cursor on cookie.
Mash numpad 5 (simulates click) and numpad 0 (simulates mouse press) and numpad + (simulates double click).
For me, mashing just 5 and 0 for a couple of seconds was enough for the achievement.
Alt+shift+numlock
Turns on MouseKeys (your numeric keypad becomes a mouse simulator).
Put cursor on cookie.
Mash numpad 5 (simulates click) and numpad 0 (simulates mouse press) and numpad + (simulates double click).
For me, mashing just 5 and 0 for a couple of seconds was enough for the achievement.