.btn {
display:block;
height: 300px;
width: 300px;
border-radius: 50%;
border: 1px solid red;
}
.btn{
border: none;
padding: 7px 26px;
text-align: center;
color: white;
background: #dc3545;
border-radius: 40px;
}
<Style TargetType="Button" x:Key="RoundButton">
<Style.Resources>
<Style TargetType="Border">
<Setter Property="CornerRadius" Value="5" />
</Style>
</Style.Resources>
</Style>