In response to Antonia, the recipe said "Bake at 350 degrees F for 20 to 30 minutes in a 9 x 13 inch greased pan. "
Some people used one cup of four instead of two, but I would check with a toothpick at around twenty minutes and go from there. A lot depends on your oven temp., have you checked it?

Often times brownies feel a little soft in the middle when you take them out, and then they firm up while cooling. I just looked at the recipe, and at the top of the page you linked, it says, "Takes 45 minutes to cook." If that's the case, it would be very odd if the center felt firm after only 30. ;o)

This is probably a dumb question, but is it possible you forgot to add the flour ? (something I've done......)