Small electric water heater below sink to get instant hot water?
I have also considered doing this, but have not so far (inertia). One way to do this is to add a separate single faucet for the electric water heater and T off the cold supply. That way when you use the hot you don't leave hot water abandoned in the piping. If you T off the hot supply, you will leave hot water in the lines when you are finished with the hot, but you will have a full flow of hotter water.

A small tank can allow you to have on tap at gallon or two of very hot water even using the 120 V circuit which is probably there to power a disposer. If like a lot of people you have removed the disposer, you could use the receptacle for this. If your disposer receptacle is on a dedicated 20-A circuit, this would work very well, but if not, you could use the disposer wall switch to turn off the heater when high power appliances are being used with other receptacles on this circuit.    

A tankless heater would have to be at least 30 A at 240 V to give a decent flow of very hot water. You would have to have add the cost and trouble of adding this circuit to the project, but this would be ideal.

EDIT

One of these very small tanks with its own spigot (presumably rated for very hot water) might be the best choice. You don't want to run very hot water (200 F) through a standard faucet.