Kristoffer's would be a nice stop for a slice of chocoflan or tres leches cake. But you are starting at 18th and Ashland, right? You need something before you get to 18th and Halsted.
What day are you planning on doing this on? and what time will you start? Here are some ideas starting at 18th and Ashland and heading east:
If you begin your trek around 9, you could get a couple tamales from La Cebollita, on Ashland just south of 18th. They have both savory and sweet tamales; the traditional drink to go with them is champurrado (hot chocolate flavored masa drink).
The fabulous Bombon bakery on 18th just east of Ashland is a must stop for something sweet.
From Nuevo Leon (open at 7 am) you could perhaps get an order of chilaquiles or machacado con huevo to go.
If you pass by Birrieria Reyes de Ocotlan after they open (10 am?) get a couple goat tacos.
Carnitas Don Pedro will definitely be open (from 6am M-Th, 5 am F-Sun) -- a quarter-pound of carnitas will keep you all going as you head east. Might want to bring some baby-wipes along...
Then stop into Kristoffer's for chocoflan or tres leches, and head north on Halsted.
Are you doing this on a Sunday? If so, Maxwell Street Market (on Desplaines north of Roosevelt) is a must.
If not on Sunday, maybe head a few blocks west on Taylor Street for beef at Al's and 'Italian lemonade' at Mario's (assuming this takes place after Mario's opens May 15).
then back to Halsted to Greektown (depending on what you want to eat, maybe just cookies at Pan Hellenic Bakery or Artopolis? Or some kalamata olives from Elea?)
From Greektown you could make your detour into the Loop. If you do this on a weekend, some food options there will not be open, though.