this recipe is nowhere near a cake in the usual sense. it sounds like something concocted by a condensed milk company's recipe staff, and being unable to come up with a better name, they called it a cake. as marmalady mentioned, it sounds more like a bar cookie(?) or an attempt at an eggless brownie.
most usually, a gooey cake center is caused by a too-high oven temp, which will start burning the top before the heat reaches the center. however 180 celsius is absolutely standard, so unless your oven thermostat is goofy (which is a possibility), that's not the likely cause.
the next likely cause would be using a smaller diameter pan than called for, thereby increasing the depth of the batter, with results similar to the above. 9" x 9" sounds about right though.
another possiblity outside of the recipe, is the kind of oven you're using. if you're using a convection oven by any chance, you should turn off the fan if possible, or decrease the temperature by between 10 20 degrees centigrade, (depending on your oven) if you can't.
looking at the recipe, all i can honestly say is that it looks lame, if you're expectling a "cake". not only the flour/ condensed milk ratio, but the amount of oil too. plus, the condensed milk already being rather sweet, i'd say the amount of sugar is is a bit high. sugar contributes not only to sweetness, but to moisture also. on top of that, i can't figure out why someone put baking soda in that recipe. there's nothing acidic in the recipe for the baking soda to react with. maybe it's there to prevent heartburn when it makes its way into your tummy.
you could try fooling around with the recipe if you're so inclined. you might try making it without the egg, as according to the recipe. it may be that after it cools, you can put it in your fridge, and it'll be a nice ... something or other. if you want to try to make it more cake-like, increase the amount of flour and sift it well with between 2 - 3 teaspoons of baking powder, no soda. reduce the sugar and the oil a bit also. instead of oil or melted butter, you might also try to cut in the butter with a pastry cutter instead. this might give you something more like a chocolate chip scone or (american) biscuit. (sounds like it might actually work this way) maybe use a little larger pan or reduce the temp. by about 10 degrees ( although they sound like they should be about right.
my honest opinion is that, if you want a cake, this recipe is beyond redemption. unfortunately, a lot of recipes are developed as a product marketing device and, through home and garden type magazines, pushed on women as "time-saving" "quick & easy" recipes by mixing together a lot of comercial products or using by mayonaise instead of and egg and some oil, etc. the results are seldom exceptional (or even good), end up costing more than necessary, and are generally nowhere near as healthy as using a few wholesome ingredients instead.
you also mentioned that you've had gooey cake results before. if you've been following the recipes, you might want to get an oven thermometer and make sure it's not the fault of the oven. good luck.