custom-mosaico-toolbar-button.js

plugins.push(function(viewModel) {
    var addMyButton = function() {
        var $toobarRightButtons = $('#toolbar .rightButtons');
        if (!$toobarRightButtons.length) {
            setTimeout(addMyButton, 50);
            return;
        }
        $('<a title="Hello" class="ui-button ui-corner-all ui-widget" role="button">\
            <span class="ui-button-icon ui-icon fa fa-fw fa-times-circle"></span>\
            <span class="ui-button-icon-space"></span>Hello\
          </a>')
        .appendTo($toobarRightButtons)
        .css('color', 'white', '!important')
        .on('click', function() {
            alert('Yay!');
        });
    };
    addMyButton();
});

If anyone is new to mosaico and node, like me, and is reading this wondering where to add this code, I've just added it to line 27 of /html/editor.html underneath plugins = [function(vm) {window.viewModel = vm;}]; (uncomment this line), and it works just fine :)