That’s controlled by a Dashicon (font icon) and a before statement:
#wpadminbar #wp-admin-bar-wp-logo > .ab-item .ab-icon:before {
content: '\f120';
top: 2px;
}
What you’d want is to replace the content call.
Use a built in Dashicon from here: http://melchoyce.github.io/dashicons/
So the smilie would be content: '\f328
;`