In a large mixing bowl, combine all-purpose flour, salt, and baking powder. Cut in softened unsalted butter until the mixture resembles coarse crumbs.

Integrate a large egg and sour cream into the mixed dry ingredients and stir until a rough dough forms.

Wrap the dough in plastic wrap and chill in the refrigerator for about 15 minutes.

In a mixing bowl, beat together two large eggs, then fold in shredded cheese and black pepper.

Retrieve the dough and roll it out to about ¼ inch thickness on a floured surface.

Cut out circles about 4 inches in diameter from the rolled dough using a round cutter.

Place a generous spoonful of filling in the center of each dough circle and fold them over.

Seal each piroshki by pinching the edges, creating a crimp if desired.

Heat oil in a large skillet over medium-high heat until it reaches 375°F (190°C).

Fry each piroshki for 3-4 minutes on each side until golden brown and crispy, or bake for 20-25 minutes.

Cool slightly on a wire rack before serving.