Try this:
<Button x:Class="sandbox.BtnLabel"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
x:Name="this">
<Button.ToolTip>
<ToolTip DataContext="{Binding Path=PlacementTarget, RelativeSource={RelativeSource Self}}">
<TextBlock Background="Yellow"
Text="{Binding LabelText}" />
</ToolTip>
</Button.ToolTip>
<TextBlock Background="Yellow"
Text="{Binding ElementName=this,
Path=LabelText}" />
</Button>
We add a ToolTip
element and assign it's DataContext
as it's PlacementTarget
which should then reach the TextBlock