In a bowl, mix the cheese and the sour cream together.
Lay out one flour tortilla and spread 1/2 of the cheese mixture on tortilla. Try to get it in the shape of a square as much as you can, because once this is built, you're going to cut the round edges off to make a square.
Then top with half the sliced pastrami.
Spread the next tortilla with some of the butter, and place the butter side down on the pastrami.
Spread remaining cheese mixture on top of the 2nd tortilla. Top with remaining pastrami. Butter 3rd tortilla and place butter side down onto 2nd layer of pastrami.
Take a cutting board and press down lightly to smooth the stack and to help form and set the stack.
Cover with plastic wrap and refrigerate 8 hours so the cheese will set.
Remove from refrigerator and on a flat surface use a serrated knife to trim edges to make a square.
Using the serrated knife, in a gentle sawing motion, cut into 6 or 7 strips, then into squares. Serve with toothpicks.