A simple attribute for Unity to display an int field in the inspector as a GameObject layer dropdown that is limited to one selection.

LayerFieldAttribute.cs

using UnityEngine;

/// <summary>
///     Allows a single GameObject layer to be selected from the inspector when applied to an
///     <see langword="int" /> field. Intended to be used when a <see cref="LayerMask" />'s
///     multiselection is not suitable.
/// </summary>
public class LayerFieldAttribute : PropertyAttribute
{
}

LayerFieldAttributeDrawer.cs

using UnityEngine;
using UnityEditor;

[CustomPropertyDrawer(typeof(LayerFieldAttribute))]
public class LayerFieldAttributeDrawer : PropertyDrawer
{
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        using (new EditorGUI.PropertyScope(position, label, property))
        {
            property.intValue = EditorGUI.LayerField(position, label, property.intValue);
        }
    }
}

So simple. Yet, so robust.