Spread a thin layer of oil on the baking tray and line with baking paper. Preheat the oven (electric cooker: 200 °C/ circulating air: 175 °C/ gas: level 3), also heat the baking tray. Mix the buffer dough with egg.
Take out the hot baking tray and brush it thinly with oil. Form 12 buffers from the buffer batter on the baking tray. Put back into the oven and bake for 15-20 minutes, turning once. Peel and coarsely grate the carrot.
Wash the lettuce and drain. Put lettuce and carrots in a bowl. Add vinegar and 1 tablespoon of oil and season with salt and pepper. Rinse the maties, dab dry and cut into pieces. Arrange potato pancakes, matie and salad on plates.
Season sour cream with salt, pepper and sugar. Put a dollop of sour cream on each Rösti. Sprinkle with pepper. Add the rest of the cream separately.