Possibly the best place in Madrid for breakfast or a midday snack: help yourself to consomme (including a selection of sherries to complement), delicious pastries and some of the best croquetas I've ever tasted. Find a place to perch around the edge of the shop (no tables). They will also pack food for a picnic. Not cheap, but managed on a student budget.
Metro Sol, c/ carrera san jeronimo 8
(note: there is also a restaurant attached)
Is this your business?