Spróbowałem tak:
Controls/Styles/NotificationCountStyle.xaml
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:Controls.Styles">
<ControlTemplate x:Key="NotificationCircleTemplate">
<Grid Background="Transparent">
<Ellipse Fill="#FFC52727" VerticalAlignment="Center" HorizontalAlignment="Center"/>
<TextBlock Text="{Binding OutputText, Mode=OneWay, UpdateSourceTrigger=PropertyChanged}" Foreground="White" Background="Transparent" VerticalAlignment="Center" HorizontalAlignment="Center"/>
</Grid>
</ControlTemplate>
</ResourceDictionary>
ModulesSelector/Styles/ModuleElementButton.xaml
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:ModulesSelector.Types">
<ResourceDictionary x:Key="DictControls">
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="/Controls:component/Styles/NotificationCountStyle.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
<ControlTemplate TargetType="{x:Type Button}" x:Key="ButtonStandard">
<Grid>
<Border CornerRadius="9">
<Grid>
<Button Width="16" Height="16" VerticalAlignment="Bottom" HorizontalAlignment="Right" Template="{StaticResource ResourceKey=NotificationCircleTemplate}"/>
</Grid>
</Border>
</Grid>
</ControlTemplate>
</ResourceDictionary>
Niestety nie rozpoznaje mi zasobu NotificationCircleTemplate.