You're trying to set both the
Content
property and the content of the element. You can't set both at the same time.
Try:
<Button Background="Transparent">
<StackPanel>
<TextBlock FontFamily="Segoe MDL2 Assets" Text="" Width="50" Height="50"/>
<TextBlock Text="Settings" FontSize="18" VerticalAlignment="Center"/>
</StackPanel>
</Button>