Custom TinyMCE Dashicon

custom-tinymce-plugin-icon.js

(function() {
	tinymce.PluginManager.add('custom_tinymce_plugin', function(editor,url) {
		editor.addButton('custom_tinymce_plugin', {
			title: 'Custom TinyMCE Plugin',
			icon: 'bold',
			onclick: function() {
				editor.insertContent('Button clicked!');	
			}
		})
	});
})();

custom-tinymce-plugin-image.js

(function() {
	tinymce.PluginManager.add('custom_tinymce_plugin', function(editor,url) {
		editor.addButton('custom_tinymce_plugin', {
			title: 'Custom TinyMCE Plugin',
			image: 'icon.png',
			onclick: function() {
				editor.insertContent('Button clicked!');	
			}
		})
	});
})();

custom-tinymce-plugin.css

i.mce-i-icon {
	font: normal 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;
}

custom-tinymce-plugin.js

(function() {
	tinymce.PluginManager.add('custom_tinymce_dashicons', function(editor,url) {
		editor.addButton('custom_tinymce_dashicons', {
			title: 'Custom TinyMCE Dashicons',
			
			/**
			* We specify two CSS classes for the TinyMCE Editor Button:
			*
			* icon: this gets prefixed with i-mce-, resulting in the first CSS class = i-mce-icon
			*
			* dashicons-star-filled: this is the WordPress Dashicon - see http://melchoyce.github.io/dashicons/ for a list of valid classes,
			* any of which can be used here.
			*/
			icon: 'icon dashicons-wordpress',
			
			// Just to prove the plugin works :)
			onclick: function() {
				editor.insertContent('Button clicked!');	
			}
		})
	});
})();

custom-tinymce-plugin.php

<?php
add_action( 'admin_enqueue_scripts', 'adminScriptsAndCSS' );
function adminScriptsAndCSS() {
	// Location of your custom-tinymce-plugin.css file
	wp_enqueue_style( 'custom_tinymce_plugin', plugins_url( 'custom-tinymce-plugin.css', __FILE__ ) );
}