Using Button.Content to set content for Button
<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" HorizontalAlignment="Center" VerticalAlignment="Center"> <Button xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"> <Button.Content> <Rectangle Height="40" Width="40" Fill="Black"/> </Button.Content> </Button> </Page>