Buck converter MOS Drive resulting in overheat
A couple of sanity check calculations: -

The inductor is a Wurth 2.2 uH and with a 300 volt DC supply, the rate at which current grows (di/dt) when the MOSFET is activated is: -

$$\dfrac{300}{2.2\mu}$$ 

This is a di/dt of 136.4 amps per microsecond.

The MOSFET is rated at peak drain current of 44 amps and it would take approximately 323 ns to reach the limit. 323 ns and an operating frequency of 20 kHz is a duty cycle of 0.65% so it looks to me like either the inductor is much too small in value or you need to run at a much higher operating frequency.