In a medium-sized mixing bowl, whisk together the first six ingredients, then cut in the butter, using a pastry blender, fork, your fingers or a mixer. Stir in the sour cream or yogurt, then enough water to make the dough cohesive. Pat or roll the dough into two 3/4-inch-thick circles, or a 3/4-inch-thick square. Cut each circle into six pie-shaped wedges; or cut the square into 1 1/2- to 2-inch squares or circles. Place the scones on lightly greased or parchment-lined baking sheets (the circles should be left intact, with the wedges separated just a bit), and bake them in a preheated 425°F oven for 15 to 18 minutes, or until they're a light, golden brown.