Preheat the oven to 375°F (190°C). Grease a 9x5 inch loaf pan and line with parchment paper for easy removal.
In a large bowl, whisk together the flour, baking powder, baking soda, salt, and sugar until evenly combined.
Cut the cold cubed butter into the dry ingredients using a pastry cutter or your fingertips until the mixture resembles coarse crumbs.
Stir in the grated cheddar until coated with the flour mixture to prevent clumping, reserving a small handful to sprinkle on top if desired.
In a separate bowl, whisk together the buttermilk, sour cream, and garlic powder. Pour the wet ingredients into the dry ingredients and fold gently until just combined — do not overmix.
Spoon the batter into the prepared loaf pan and smooth the top. Sprinkle the reserved cheddar over the batter.
Bake for 22 to 28 minutes, or until the top is golden and a toothpick inserted into the center comes out clean. If the top browns too quickly, tent with foil.
Brush the warm loaf with melted butter and sprinkle with chopped parsley. Allow the loaf to cool in the pan for 10 minutes before transferring to a wire rack to cool slightly. Serve warm, sliced.
Notes
Serve warm. For best texture, grate your own cheddar and use cold butter. Leftovers keep well wrapped at room temperature for 1 day or refrigerated for 3 days.