Chocolatería San Ginés is an institution in Spain. I've been travelling to Madrid for 20 years and I always end up there at some point. You can go any time you want, it's open 24 hours. Usually we end up there after a several drinks. Churros con chocolate are perfect for soaking up the booze and work well as a dessert or the traditional breakfast too. I remember going there at 5 in the morning one time and it was rammed with people of all ages. This last time, we visited San Ginéstwice in three days. Yes, it's very popular with tourists but I don't care, I'll always make a visit every time we're in Madrid, it's tradition.