Simit is a bakery product that looks like a tiny bagel. Although eating it for breakfast is the most common, you can have it anytime you want to appease your hunger. It can be found in most bakeries and also at mobile food counters on the streets.
I want to mention that this spot is another old resident of the Taksim region. This simit bakery has been running since 1977 and they are still doing pretty well.
If you are staying in Taksim, here is a great breakfast plan for you: take a morning walk to Bogazkesen Simit bakery, grab a couple of simits and keep walking down to the shore. Bogazkesen street you will be walking on is also full of design shops and hipster cafes, which will make your walk fun. When you are getting close to the seaside, you will see street counters on your right squeezing fresh orange juice and serving tea. There you have a perfect pairing: tea and simit. Now you can find a bench at the seashore and enjoy your tiny breakfast!