* Just Desserts activates when your opponent Declares an attack.
* Declaring an attack is a different action than announcing an XYZ or Synchro Summon.
Here's how the timing works:
1. Opponent announces their XYZ or Synchro Summon: This is a separate action from declaring an attack.
2. Opponent completes their Summon: They complete the materials needed for the summon.
3. Opponent declares an attack: This is the point where you could potentially activate Just Desserts.
In short, you can't activate Just Desserts in response to an XYZ or Synchro Summon because they are distinct actions from declaring an attack.