“Ah! D-arling! Please! I-I-can’t take it!” Mettaton pants. You take this as one part challenge, one part revenge. The heart is malleable, and if you're slow and gentle enough, you can slide your fingers into it. Which is exactly what you do. You're about two knuckles deep before you start flexing and crooking your fingers inside the heart, still absolutely eating him out with all the energy your post-vibrator body can muster.
“Ah! Uh- Ah! Ah-ahh! D-ARLING!” Mettaton shrieks. You giggle and keep giving his heart little kitten licks until his moans turn to little glitchy pants.