"I need to remember that, my dear - and when I return home, pester you into making me some." Gen smiled slyly.

He couldn't think of the fact that Senku might be dying, or dead, at the time he'd been taken from.

So, instead, he cheekily popped the spoon into his own mouth after getting more ice cream - the real reason behind his crafty expression. He knew exactly what he was doing, but doubted Senku picked up on it. Since this was harmless? That was fine.