not all cocoa is just cocoa. there are two types, natural and dutch process.
natural cocoa is acidic, and calls for baking soda.
dutch process cocoa is treated with alkali, so recipes using dutch process cocoa need baking powder.
your recipe calls for 2 teaspoons of baking soda. if you're using dutch instead of natural cocoa, it won't rise.
another possible problem is that 2 teaspoons of soda is an extreme amount if using natural cocoa, because soda is 3 or 4 time more powerful than baking powder (in other words, soda is only 1/4 to 1/3 of the volume in baking powder). if this is the case, then your cakes will be over-rising, making the bubble walls too thin to continue to support the weight, hence collapsing.
i'm just going to take a stab at amounts here, but i'd quess that about a tablespoon of baking powder if using dutch process cocoa or, about a 1/2 teaspoon of soda or a bit more if using natural cocoa.
actually, looking at the recipe again, i see that the cocoa is only 1/4 the amount of the flour, which is not really a lot, so i'd be inclined to suggest a mix of about 1 to 1 1/2 teaspoons powder, and 1/4 to 1/2 teaspoon soda.
if you continue to have problems, you could also try not removing the cakes immediately from the oven, but turning off the heat, cracking the oven door open a couple of inches, and give them some time to cool down more slowly. this sometimes works for cakes that are a little over-risen.
another possiblity that pops into mind would be the amount of flour. if you perhaps used to sift the flour after
measuring, but are now measuring already
sifted flour, you'll actually be using a lot less flour. the batter will be more runny and weak.