In a large mixing bowl, combine flour, baking powder, salt and 1 tablespoon of sugar.
Mash the farmer's cheese very well (we like to push it through a fine mesh sieve to ensure no large lumps remain). Alternatively, blitz it with a stick blender.
Place the flour mixture on a large cutting board or a cut-proof kitchen surface. Add the cubed butter to the flour mixture, and using a long-bladed knife, chop the mixture until it resembles coarse breadcrumbs.
Scatter the farmer's cheese on top, and chop and cut the mixture to incorporate.
Add the sour cream and chop and cut again until it starts coming together into a dough. If the mixture is too dry, add a little more sour cream.
Bring the dough together into a circle, kneading just enough for it to come together - don't overwork the dough. Wrap it in a large piece of plastic wrap (cling film) and roll it to flatten it further - it will make it easier to roll out thinly later. Place it in the fridge for 2-4 hours.
When ready to shape the cookies, beat an egg in a small bowl.
In another bowl, combine sugar and cinnamon.
At this point, preheat the oven to 220°C (420°F). Line a couple of large cookie sheets with parchment paper.
Divide the dough into 2-4 portions (this will make it more manageable to roll out). Place the portions of dough that you are not immediately using back in the fridge.
Roll the dough out thinly (no more than ⅛ inch) on a lightly floured work surface.
Using a 3-inch (8 cm) cookie cutter (or a thin rimmed cup or glass), stamp out the circles of dough. The scraps can be rerolled (but aim to cut the circles close to each other to avoid much rerolling - these cookies will be tougher).
Brush each circle with a little beaten egg. Scatter cinnamon sugar on top of the egg-washed side of the circle, then fold it in half. Brush the surface with egg and sprinkle more cinnamon sugar. Then fold it over again. You should now have a folded triangle (ish).
Brush the top of the cookie with more egg and sprinkle a little more cinnamon sugar.
As you go, arrange the farmer's cheese cookies on the prepared baking sheet, spacing them out (about 1.5 inches apart).
Bake the cookies for 10-12 minutes, until golden brown and puffed up slightly.
TIP: It is best to keep the dough cold before it goes into the oven - this creates the flakiest cookies. We, therefore, recommend baking the prepared cookies whilst you roll out the other portion of the dough and shape the next batch of cookies.
Once baked, place them on a wire rack to cool down!