type
  TButton = class(Vcl.StdCtrls.TButton)
  protected
    procedure CreateWnd; override;
  end;

implementation

{ TButton }

procedure TButton.CreateWnd;
begin
  inherited;
  // note, that the handle to the region passed to this function should be stored
  // somewhere and released when is no longer needed by the DeleteObject function
  SetWindowRgn(Handle, CreateEllipticRgn(0, 0, ClientWidth, ClientHeight), True);
end;