code: function(cls) {
          cls = cls || '';
          var me = this,
              btnIconEl = me.btnIconEl,
              oldCls = me.iconCls || '';
              
          me.iconCls = cls;
          if (oldCls != cls) {
              if (btnIconEl) {
                  // Remove the previous iconCls from the button
                  btnIconEl.removeCls(oldCls);
                  btnIconEl.addCls(cls || '');
                  me.setComponentCls();
                  if (me.didIconStateChange(oldCls, cls)) {
                      me.updateLayout();
                  }
              }
              me.fireEvent('iconchange', me, oldCls, cls);
          }
          return me;
      }
docs: Sets the CSS class that provides a background image to use as the button's icon. This method also changes the
  value of the {@link #iconCls} config internally.
  @param {String} cls The CSS class providing the icon image
  @return {Ext.button.Button} this