#if UNITY_EDITOR
[UnityEditor.CustomEditor(typeof(YourScriptName))]
public class YourScriptNameEditor : UnityEditor.Editor
{
    public override void OnInspectorGUI()
    {
        DrawDefaultInspector();

        var yourScriptNameInstance = (YourScriptName)target;
        if (GUILayout.Button("Button"))
        {
            yourScriptNameInstance.YourMethod();
        }
    }
}
#endif