example.tsx

class Content extends React.Component {
  constructor() {
   super(); 
  }
  
  render() {
    return (
      <Fabric className='foo'>
        <DefaultButton
          onClick= {() => alert('hello')}
          onRenderText= {this._onRenderText}
          text="Hi There"
          foo="Foo"
        />
      </Fabric>
    );
  }
         
  private _onRenderText:IButtonProps['onRenderText'] = (props, defaultRender) => {
    return (
      <span> {defaultRender()} {props.foo} </span>
    );
  }
}

example2.tsx

class Content extends React.Component {
  constructor() {
   super(); 
  }
  
  render() {
    return (
      <Fabric className='foo'>
        <DefaultButton
          onClick= {() => alert('hello')}
          onRenderText= {this._onRenderText}
          text="Hi There"
          foo="Foo"
        />
      </Fabric>
    );
  }
         
  private _onRenderText:IRenderFunction<IButtonProps> = (props, defaultRender) => {
    return (
      <span> {defaultRender()} {props.foo} </span>
    );
  }
}

From https://twitter.com/nickytonline/status/981301072697360390

I'm sure there's more to your class, but if it's something simple like this, it could just be a React.SFC<IButtonProps> and _onRenderText could be a utility function since it does not use this.

// import IButtonProps and IRenderFunction

class Content extends React.Component<IButtonProps, {}> {  
  render() {
    return (
      <Fabric className='foo'>
        <DefaultButton
          onClick= {() => alert('hello')}
          onRenderText= {this._onRenderText}
          text="Hi There"
          foo="Foo"
        />
      </Fabric>
    );
  }
       
  private _onRenderText:IRenderFunction<IButtonProps> = (props, defaultRender) => {
    return (
      <span> {defaultRender()} {props.foo} </span>
    );
  }
}