Custom control OnApplyTemplate called after dependency property callback

I'm developing my first WPF custom control and I'm facing some problems, here's a simplified version of the code I'm currently using:

using System.Windows;
using System.Windows.Controls;

namespace MyControls
{
    [TemplatePart(Name = "PART_Button", Type = typeof (Button))]
    public class MyControl : Control
    {
        public static readonly DependencyProperty ContentProperty = DependencyProperty.Register("Content", typeof (object), typeof (MyControl), new PropertyMetadata(null, OnLabelPropertyChanged));

        private Button _buttonElement;

        public object Content
        {
            get { return this.GetValue(LabelProperty); }
            set { this.SetValue(ContentProperty, value); }
        }

        static MyControl()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof (MyControl), new FrameworkPropertyMetadata(typeof (MyControl)));
        }

        private static void OnContentPropertyChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
        {
            MyControl myControl = sender as MyControl;
            if (myControl != null && myControl._buttonElement != null)
                myControl._buttonElement.Content = e.NewValue;
        }

        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();

            this._buttonElement = this.Template.FindName("PART_Button", this) as Button;
        }
    }
}

This is the template for my custom control:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:local="clr-namespace:MyControls">
    <Style TargetType="{x:Type local:MyControl}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type local:MyControl}">
                    <Button x:Name="PART_Button" />
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>

Then i put it inside a Grid and try to set its Content property:

<Grid x:Name="layoutRoot">
    <controls:MyControl x:Name="myControl" />
</Grid>

Here's the code behind:

using System.Windows;

namespace MyControls
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            this.InitializeComponent();

            this.myControl.Content = "test";
        }
    }
}

This doesn't work, for some reason the OnContentPropertyChanged callback is called before OnApplyTemplate, so myControl._buttonElement is assigned too late and it's still null when trying to set its content. Why is this happening and how can I change this behavior?

I also need to provide full design time support but I cannot find a way to make my custom control accept some additional markup, much like the Grid control does with ColumnDefinitions:

<Grid x:Name="layoutRoot">
    <Grid.ColumnDefinitions>
        <ColumnDefinition />
        <ColumnDefinition />
    </Grid.ColumnDefinitions>
</Grid>

Any help would be greatly appreciated!

UPDATE

I found a document that explains why the OnApplyTemplate method is called after control properies are set:

http://msdn.microsoft.com/en-us/library/dd351483%28v=vs.95%29.aspx

So the question is: how can I keep track of the properties that are set (in XAML or programmatically) and methods that are called when the control has not been initialized, so that I can set/call them when the OnApplyTemplate method is called? How can the same callback/method work both before and after the control initialization without duplicating the code?