Unity Single layer dropdown from this forum post https://answers.unity.com/questions/609385/type-for-layer-selection.html

LayerAttribute.cs

using UnityEditor;

/// <summary>
 /// Attribute to select a single layer.
 /// </summary>
 public class LayerAttribute : PropertyAttribute
 { 
      // NOTHING - just oxygen.
 }

LayerAttributeEditor.cs

using UnityEditor;
using UnityEngine;

[CustomPropertyDrawer(typeof(LayerAttribute))]
 public class LayerAttributeEditor : PropertyDrawer
 {
     public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
     {
         EditorGUI.BeginProperty(position, label, property);
         property.intValue = EditorGUI.LayerField(position, label, property.intValue);
         EditorGUI.EndProperty();
     }
 }