[stil]

Код:

var butRgn: HRGN;
 points: array [0..6] of TPoint;
begin
with Button1 do begin
  points[0].X := width div 2;
  points[0].Y := 1;
  points[1].X := width-1;
  points[1].Y := height div 2;
  points[2].X := width div 2;
  points[2].Y := height-1;
  points[3].X := width div 2;
  points[3].Y := height-height div 4;
  points[4].X := 1;
  points[4].Y := height-height div 4;
  points[5].X := 1;
  points[5].Y := height div 4;
  points[6].X := width div 2;
  points[6].Y := height div 4;
end;
butRgn:=CreatePolygonRgn(points, 7, WINDING);
SetWindowRgn(Button1.Handle, butRgn, true);

Вот может подойдет