Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.

In a large bowl, whisk together flour, dill, baking powder, sugar, salt, and pepper. Using a pastry blender or 2 forks, cut in cold butter until mixture is crumbly.

In a small bowl, whisk together potatoes, cold sour cream, and ½ cup (120 grams) ice water. Add potato mixture to flour mixture, stirring with a fork just until mixture starts to come together. Using your hands, knead dough just until combined.

Turn out dough onto a lightly floured surface, and roll to 1-inch thickness. Using a 2½-inch round cutter dipped in flour, cut dough without twisting cutter, rerolling scraps once, and place at least 1 inch apart on prepared pan. Brush tops of scones with egg.

Bake until golden brown, 20 to 25 minutes. Let cool on pan for 5 minutes. Serve warm with sour cream.