Preheat the oven to 200°C (approximately 400°F) convection.
In a bowl, miix together the flour, baking powder, sugar, salt, butter and yogurt and knead until the dough comes together. On a floured surface, roll out the dough 2 cm (approximately 3/4-inch) thick. Cut 4 cm (approximately 1 1/2-inch) circles and place on the baking sheet, spacing them 2 inches apart. Mix the egg yolk with 1 tablespoon water and brush on the circles. Bake until golden brown, about 15 minutes. Remove from the oven and cool on a wire rack.