This can be done by overwriting the Control Template. Here is a very rough example to get you started.
EDIT Updated Path to be L shape
<Button >
<Button.Template>
<ControlTemplate>
<Border>
<Grid>
<Polygon Points="300,200 200,200 200,300 250,300 250,250 300,250" Fill="Purple" />
</Grid>
</Border>
</ControlTemplate>
</Button.Template>
</Button>
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solved WPF How can i make a button in “L” form