Say you are on earth observing a staring hypernova at safe distance from earth.

Now as soon as the core collapses to its Schwarzschild radius the singularity would come into existence and so would it's event horizon which at the moment of formation of singularity would be very small.

Now as any more matter stats to fall in it would appear to slow down and take infinite time to fall through for an observer at earth.

So for the observer the black hole would never get larger than a infinitesimal point at the center of the star.

So what do we see that astronomers claim as blackhole?

The only explanation I can think of is a cascading effect.

I.e the event horizon of the singularity will always be larger than the singularity so as soon as the horizon forms some matter would end up inside it from the start just because the matter was at lesser distance from the singularity than the radius of event horizon at the moment singularity formed. This would make the event horizon grow again due to the mad that just ended up inside it. Now again some matter would end up inside the new radius of the event horizon. This would continue like a chain reaction

I don't know how much sense that makes but it looks like the only way a black hole can form in finite time for a distant observer