What is ideal temperature for baking cupcakes?
The most common reason for cupcakes to sink is an oven temperature that is too high. 180C is equivalent to 356F. Most recipes in the US for cupcakes call for an oven temperature of 350F. 180 and 350 are just round numbers signifying a moderate oven. Cupcakes are picky though, a very slight difference in temperature can make a big difference in the final cupcake. So try dropping your temperature a bit, and be sure to use an oven thermometer.

Opening the door during baking can cause sinking, so don't do that! (I see that you didn't, so that's one possibility down)

Use fresh baking powder. Baking powder that has been opened for just a matter of a few months starts losing effectiveness. Baking soda on the other hand, lasts pretty much forever if stored properly.

Are you by any chance using Dutch processed cocoa? If so, try adding a small amount (a quarter teaspoon or so) of white vinegar. Baking soda needs acid to work properly, and Dutch processed cocoa has been made nonacidic. Since your recipe also includes baking powder, that is unlikely to be the problem unless your baking powder is stale.

Finally, follow the recipe carefully regarding beating. Overbeating the batter can cause sinking.

That's the best answer I can give based on your limited information. We can help you narrow it down further if you give us every little bit of information you can.