Мое решение будет (но, конечно, граница с радиусом угла проще):
<Border Margin="0,15,0,0" Height="75" Width="75">
<Grid Width="{Binding Path=ActualWidth, RelativeSource={RelativeSource PreviousData}}"
Height="{Binding Path=ActualHeight, RelativeSource={RelativeSource PreviousData}}" >
<Ellipse Width="{Binding Path=Width, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=Border}}"
Height="{Binding Path=Height, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=Border}}"
Stroke="White" StrokeThickness="1"/>
<Border Width="{Binding Path=Width, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=Border}}"
Height="{Binding Path=Height, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=Border}}"
VerticalAlignment="Center" HorizontalAlignment="Center">
<TextBlock HorizontalAlignment="Center" VerticalAlignment="Center" Margin="0,0,1,1">
<StackPanel Orientation="Vertical" HorizontalAlignment="Center" >
<TextBlock Foreground="White" FontSize="18" FontFamily="{StaticResource RobotoFont}"
FontWeight="DemiBold"
HorizontalAlignment="Center" Text="0"/>
<TextBlock Foreground="White" FontSize="12" FontFamily="{StaticResource ResourceKey=RobotoFont}" FontStyle="Normal"
HorizontalAlignment="Center" Text="Selected"/>
</StackPanel>
</TextBlock>
</Border>
</Grid>
</Border>