The simplest solution without changing the default style is:
<Style TargetType="Button" x:Key="RoundButton">
<Style.Resources>
<Style TargetType="Border">
<Setter Property="CornerRadius" Value="5" />
</Style>
</Style.Resources>
</Style>
Then just define your button like this:
<Button Style="{DynamicResource RoundButton}" />