A minor, cosmetic, improvement (FrameMargins -> None):
anima = False;
Manipulate[
If[anima, ParametricPlot[{Cos[t], Sin[t]}, {t, 0, 2 Pi}], ""],
Button["Iniciar/Detener", If[anima, anima = False, anima = True]],
AppearanceElements -> None, FrameMargins -> None
]