Of all the ways one can go on a diet, I would humbly suggest that you consider adopting the High-Sugar-High-Fat (HFHS) diet that I just invented. It's very likely to make you fat as a blimp and shorten your life by decades; but the upside is that if someone asks you if you "shouldn't be on a diet?", you can answer "I am". The best place to go on a HFHS diet is at Zanoni & Zanoni, half-way between Vienna's Stephansdom and Schwedenplatz.
Kidding aside, Zanoni & Zanoni is really a fantastic place to get something to sink your sweet tooth into. The atmosphere is best described as "chaotic from the outside" - but the service is very fast, so there has to be a method to the madness. The dishes are, as can be expected, fantastic. Gaining weight has never been such a pleasure. My favorite is the Erdbeer Crepe, a vanilla and strawberry ice-cream covered in strawberries, with a rolled up crepe filled with strawberry sauce and whipped cream on the side.
Generous opening hours, fast service and sweet, sweet ice cream dishes makes this a place I only need half an excuse to visit. While there are many ice cream restaurants on the Schwedenplatz, Zanoni & Zanoni is the one I keep coming back to.