Using a dashicon for a custom button in TinyMCE?

Here’s your easy solution (hopefully this will help other people too):

1) add a custom class for icon, in this example “myicons”

(function () {
    tinymce.PluginManager.add('twitter_button_plugin', function (editor, url) {
        editor.addButton('mce_tweet_button', {
            title: 'Insert tweet',
            icon: 'myicons dashicons-twitter',
            onclick: function() {
                ...
            }
        });
    });
})();

2) Enqueue your admin stylesheet file

function load_custom_wp_admin_style() {
   wp_enqueue_style( 'custom_wp_admin_css', 'URL/TO/custom_admin_style.css' );
}
add_action( 'admin_enqueue_scripts', 'load_custom_wp_admin_style' );

3) Then in your custom_admin_style.css file add this:

/**
 * TinyMCE add support for dashicons
 */
i.mce-i-myicon {
    font: 400 20px/1 dashicons;
    padding: 0;
    vertical-align: top;
    speak: none;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
    margin-left: -2px;
    padding-right: 2px
}