Node.js - wait for process.exit() to execute after earlier code is finished

In my node.js application I want to write some data to a logfile when the server is shutdown (thus when CTRL+C has been done in the cmd). The problem is that the process.exit() is called before the writing to the file is finished. I tried using a callback and jQuery $.Deferred.resolve(), but to no avail: probably because the file-write is async but I'd like to keep it asynchronous.

The callback code:

if (process.platform === "win32"){
    var rl = readLine.createInterface ({
        input: process.stdin,
        output: process.stdout
    });

    rl.on ("SIGINT", function (){
        process.emit ("SIGINT");
    });

}

process.on ("SIGINT", function(){
    var stopServer = function() {
        //this happens way too early, the logger.log has not written it's data yet
        process.exit();
    };

    var logServerStop = function(callback) {
        logger.log("SERVER SHUTDOWN: ", true);
        logger.log("-----------------------------------------");
        logger.log("");
        callback();
    };
    logServerStop(stopServer);  

});

And the logger.log code:

var fs = require('fs');
var filename = './output/logs/logfile.txt';

exports.log = function(data, addDate){
    if (typeof addDate === 'undefined') { myVariable = false; }
    var now = new Date();
    var date = now.getDate() + "-" + (now.getMonth() + 1) + "-" + now.getFullYear();
    var time = now.getHours() + now.getMinutes();

    if(addDate){
        data = data + date + " " + now.toLocaleTimeString();
    }
    var buffer = new Buffer(data + '\r\n');

    fs.open(filename, 'a', function( e, id ) {
        if(e){
            console.log("Foutje: " + e);
        }
        else{
            fs.write( id, buffer, 0, buffer.length, null, function(err){
                if(err) {
                    console.log(err);
                } else {
                    console.log("De log file is aangevuld.");
                }
            });
        }

    });
};

I'd also like to keep the log-function as it is (so I wouldn't like having to add a callback-function parameter, I'd like my problem to be handled in the callback code. Thanks in advance.

Edit 1

process.on ("SIGINT", function(){
    logger.log("SERVER SHUTDOWN: ", true);
    logger.log("-----------------------------------------");
    logger.log("", false, function(){
        process.exit();
    });

});

And the logger.log changes:

exports.log = function(data, addDate, callback){
    if (typeof addDate === 'undefined') { myVariable = false; }
    var now = new Date();
    var date = now.getDate() + "-" + (now.getMonth() + 1) + "-" + now.getFullYear();
    var time = now.getHours() + now.getMinutes();

    if(addDate){
        data = data + date + " " + now.toLocaleTimeString();
    }
    var buffer = new Buffer(data + '\r\n');

    fs.open(filename, 'a', function( e, id ) {
        if(e){
            console.log("Foutje: " + e);
        }
        else{
            fs.write( id, buffer, 0, buffer.length, null, function(err){
                if(err) {
                    console.log(err);
                } else {
                    console.log("De log file is aangevuld.");
                }
            });
        }

    });
    if(typeof(callback)=='function'){ callback(); }
};