<!-- DataContext for UserControl is set in code behind -->

  <Grid Margin="10">
	  <Grid.RowDefinitions>
		  <RowDefinition Height="Auto" />
		  <RowDefinition Height="*" />
		  <RowDefinition Height="Auto" />
	  </Grid.RowDefinitions>

	  <Grid Grid.Row="2"
			Margin="0 10 0 0">
		  <Grid.ColumnDefinitions>
			  <ColumnDefinition Width="*" />
			  <ColumnDefinition Width="Auto" />
			  <ColumnDefinition Width="*" />
		  </Grid.ColumnDefinitions>

		  <Button Content="DBG"
				  Height="50"
				  Width="50" 
				  Command="{Binding DebugCommand}"/>

		  <Border CornerRadius="8"
				  BorderThickness="1"
				  BorderBrush="Silver"
				  Background="DimGray"
				  Grid.Column="1"
				  Padding="0 5">
				  
			  <ItemsControl Items="{Binding HotBar.Slots}">
				  <ItemsControl.ItemsPanel>
					  <ItemsPanelTemplate>
						  <StackPanel Orientation="Horizontal" />
					  </ItemsPanelTemplate>
				  </ItemsControl.ItemsPanel>
				  <ItemsControl.ItemTemplate>
					  <DataTemplate DataType="{x:Type Slot}">
						  <Border Height="50"
								  Width="50"
								  Margin="5 0"
								  Background="Gray"
								  BorderThickness="1"
								  BorderBrush="Silver"
								  ToolTip="{Binding Content.ItemName}"
								  CornerRadius="5" />
					  </DataTemplate>
				  </ItemsControl.ItemTemplate>
			  </ItemsControl>
		  </Border>
	  </Grid>
</Grid>

[NOESIS] Binding failed: Path=Content.ItemName, Source=BindingExpression(''), Target=Border(''), TargetProperty=ToolTipService.ToolTip

public class MainViewModel : NotifyObject
{
	#region Commands
	public ICommand DebugCommand => new RelayCommand(
		_ =>
		{
			var pickaxeItem = Database.Get().Get<PickaxeItem>("DEBUG_PICKAXE");
			var itemStack = new ItemStack(pickaxeItem, 25);

			HotBar.Set(0, itemStack);
		});
	#endregion

	#region Properties
	public HotBar HotBar { get; } = new();
	#endregion
}

public class HotBar : NotifyObject
{
	public ReadOnlyObservableCollection<Slot> Slots { get; }

	private readonly ObservableCollection<Slot> m_Slots = new();

    public HotBar()
    {
		Slots = new(m_Slots);

		for (int i = 0; i < 10; i++)
		{
			m_Slots.Add(new(i));
		}
    }

    public void Set(int index, ItemStack stack)
	{
		Slots.ElementAt(index).Content = stack;
	}

	public void Clear(int index)
	{
		Slots.ElementAt(index).Content = null;
	}
}

public class Slot : NotifyObject
{
	public int Index => m_Index;
	public ItemStack Content
	{
		get => GetValue<ItemStack>();
		set => SetValue(value);
	}

	private readonly int m_Index;

    public Slot(int index)
    {
		m_Index = index;
    }
}

Cant't solve PropertyPath: Type 'BindingExpression' does not contain a property 'Content'

<ItemsControl ItemsSource="{Binding HotBar.Slots}">