Preheat the oven to 180°C (350°F). Grease a 20x20 cm (8x8 in) baking pan with a thin layer of Sunflower Oil and line the bottom with parchment paper for easy removal.
In a large bowl, smooth the Tvorog with a fork or whisk until creamy. Beat in the Eggs one at a time, then stir in the Granulated Sugar, Vanilla Sugar and a pinch of Salt until the mixture is glossy and combined.
Add the Sour Cream to the curd mixture and mix until even; this keeps the filling tender and moist. Stir in a splash of the reserved Sunflower Oil if the mixture seems too stiff.
Sift the All-Purpose Flour into the bowl and gently fold it in until you have a thick but spoonable batter—do not overmix to keep the texture light.
Spoon half of the batter into the prepared pan and spread it evenly. Drop spoonfuls of Berry Jam over the surface and use a knife to gently swirl it into the batter. Spread the remaining batter over the swirled layer, then add a few more dollops of Berry Jam on top and swirl lightly for a marbled effect.
Bake for 35–45 minutes, or until the edges are set and the center is only slightly jiggly; a toothpick inserted in the center should come out with a few moist crumbs but not wet batter. If the top browns too quickly, tent loosely with foil.
Allow the dessert to cool in the pan, then chill in the refrigerator for at least 1 hour to firm up and develop flavor. Before serving, dust the top with Powdered Sugar and cut into squares.
Serve chilled or at room temperature with extra Sour Cream or warmed Berry Jam on the side for spooning over each piece. Store leftovers covered in the refrigerator for up to 3 days.
