Code:
<Window x:Class="WPF.MainWindow"
        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"
        xmlns:local="clr-namespace:WPF"
        mc:Ignorable="d"
        Title="Test Rounded button" Height="350" Width="525">
    <Window.Resources>
        <Style TargetType="{x:Type Button}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type Button}">
                        <Grid>
                            <Border BorderBrush="Black"
                                    BorderThickness="5"
                                    Background="Yellow"
                                    CornerRadius="20">
                                <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center"></ContentPresenter>
                            </Border>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="35"></RowDefinition>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"></ColumnDefinition>
        </Grid.ColumnDefinitions>
        <Button Grid.Column="0" Grid.Row="0" Name="btnTest" Content="Rounded button" Width="200"></Button>
    </Grid>
</Window>