Are there any variable temperature electric kettle which boil water before letting temperature drop to the desired temperature?
There are some appliances that do what you're asking for, but they're not exactly kettles.  They have a reservoir of water that gets heated, and then they hold that temperature, and they have a button to dispense hot water as needed.  Most have an option to boil the water first.

As best I can tell from looking at them on amazon, they're labeled as "Water boiler and heater", but you might also try "hot water dispenser"

They're rather useful if you drink a lot of tea through the day ... you can fill them up in the morning, and then you'll have hot water at your preferred temperature when you're ready for your next cup.