WPF MenuItem Header and HeaderTemplate

I want to bind a list of KeyValuePair to a list of MenuItems. I thought I should use MenuIten.HeaderTemplate, but it didn't work. I only got blank headers.

            <MenuItem 
                Header="Template" 
                ItemsSource="{Binding Path=Samples}">
                <MenuItem.ItemTemplate>
                    <DataTemplate>
                        <MenuItem>
                            <MenuItem.HeaderTemplate>
                                <DataTemplate>
                                    <StackPanel Orientation="Horizontal">
                                        <TextBlock Text="{Binding Path=Key}" FontWeight="Bold"/>
                                        <TextBlock Text="{Binding Path=Value}" FontStyle="Italic" Margin="5,0,0,0"/>
                                    </StackPanel>
                                </DataTemplate>
                            </MenuItem.HeaderTemplate>                            </MenuItem>
                    </DataTemplate>
                </MenuItem.ItemTemplate>
            </MenuItem>

Then I replaced MenuItem.HeaderTemplate with MenuItem.Header, it worked.

            <MenuItem 
                Header="Template" 
                ItemsSource="{Binding Path=Samples}">
                <MenuItem.ItemTemplate>
                    <DataTemplate>
                        <MenuItem>
                            <MenuItem.Header>
                                <StackPanel Orientation="Horizontal">
                                    <TextBlock Text="{Binding Path=Key}" FontWeight="Bold"/>
                                    <TextBlock Text="{Binding Path=Value}" FontStyle="Italic" Margin="2,0,0,0"/>
                                </StackPanel>
                            </MenuItem.Header>