Add cold, cubed butter and mix it in the flour until you obtain a sandy mixture.
Add the cheese and the sour cream and mix until you obtain a stiff dough.
If the dough is too sticky, dust it with a teaspoon of flour and lightly knead it to incorporate the flour.
Wrap dough in foil and place it in the refrigerator for an hour.
Preheat oven to 180C/350F.
Take the dough out and split it in two parts.
Cover a baking tray with parchment paper.
Roll out the dough on the parchment paper, aprox ⅛ of an inch thick(3mm).
Use a pizza cutter to slice the dough sheet in squares, then in triangles.
Pinch each triangle with a fork.
Beat one egg and brush each triangle of dough with it.
Sprinkle poppy seeds, sesame seeds or caraway over the dough and bake the crackers at 350F/180C for 15 minutes until golden brown.
Repeat the process with the second piece of dough.
Notes
Storage:
At Room Temperature: Store the cooled crackers in an airtight container for up to 5 days.
Freeze the Dough: Wrap the dough tightly in plastic wrap and place it in a freezer-safe bag. Freeze for up to 3 months. Thaw overnight in the refrigerator before rolling and baking.
Freeze the Baked Crackers: Allow the crackers to cool completely before freezing them in airtight freezer bags or containers for up to 3 months.