Here is the current code I am using:

<UserControl
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:Class="ButtonPrototype.MainPage"
    Width="640" Height="480">
    <UserControl.Resources>
        <ControlTemplate x:Key="CellTemplate" TargetType="Button">
            <Grid>
                <Border x:Name="CellBorderBrush" BorderBrush="Black" BorderThickness="1">
                    <ContentPresenter
                      Content="{TemplateBinding Content}"  
                      HorizontalAlignment="Center"
                      VerticalAlignment="Center"/>
                </Border>
            </Grid>
        </ControlTemplate>

        <Style x:Key="CellStyle" TargetType="Button">
            <Setter Property="Template" Value="{StaticResource CellTemplate}"></Setter>
            <Setter Property="Foreground" Value="Black"></Setter>
            <Setter Property="FontSize" Value="80"></Setter>
            <Setter Property="Width" Value="100"></Setter>
            <Setter Property="Height" Value="100"></Setter>
        </Style>
    </UserControl.Resources>

    <Grid x:Name="LayoutRoot" Background="White">
        <Button Content="A" Style="{StaticResource CellStyle}"></Button>
    </Grid>
</UserControl>

The Horizontal aligning works but the verticalalignment doesn't do anything. Thanks for your help.