Preheat your oven to 200ºC/400°F.: You should notice the oven warming steadily, and when it reaches temperature the immediate benefit is even roasting from the moment the tray goes in. A properly preheated oven ensures the surface begins to crisp rather than steam. If the oven is cooler than stated, the potatoes will take longer and may steam, creating a soggy exterior instead of the desired golden crunch. Avoid opening the door frequently while preheating, which causes temperature drops and uneven heat distribution.
Place a potato on the chopping board and lay a wooden spoon handle either side. Cut slices into the potato, using the spoon handles to stop the knife cutting all the way through.: As you slice, you will hear the soft whisper of knife meeting skin and feel a slight resistance as the blade approaches the spoon handles, which act as stoppers. This technique makes the potato open into a comb like structure that crisps amazingly in the oven. The why here is surface area, more edges mean more crunchy caramelization. A common mistake is pressing too hard and cutting through, which makes the potato fall apart in the pan, so take controlled, even slices and keep your fingers safe.
Turn the potato and cut in the opposite direction to create the squares.: When you rotate and slice the opposite way you form a grid of little squares, and you will notice the potato beginning to fan slightly at the cuts. These small windows allow fat and seasoning to nestle inside, giving flavor to the interior as well as the exterior. The scent of raw potato and herb will start to mingle as you work, promising savory payoff. Avoid cutting irregularly deep lines, because uneven pieces will cook at different rates; aim for consistency for uniform doneness.
Repeat this cutting process with the remaining potatoes.: As you repeat the technique the rhythm becomes meditative, the texture of each potato yielding to your knife. Keeping the sizes consistent is important so they roast evenly and finish at the same time. If you rush this step you might end up with uneven scoring, leading to some pieces overcooked and others underdone. Take your time and treat each potato the same way for predictable results.
Mix the melted butter, olive oil, dried rosemary, garlic powder, salt and pepper together in a bowl. Stir to combine.: You will feel the warmth of the butter and see the sheen as the olive oil emulsifies, while the dried rosemary releases aroma as you stir. This flavored fat is the vehicle that carries taste into every crevice created by scoring. The reason for combining here is to ensure even coating, which promotes uniform browning. A common error is adding cold butter straight from the fridge, which can clump and not blend with the oil; warm the butter slightly so it mixes smoothly.
Add in the potatoes and stir until the potatoes are covered in the butter.: As you toss, listen for a soft patter, and notice the shiny glaze forming over each scored surface. The coating fills the cuts, so when the potatoes roast the fats baste the flesh and encourage crisp edges. This step is key to getting flavor into the center rather than only on the outside. Avoid overcrowding the bowl which prevents even coating, and be gentle so the scored potatoes do not break apart.
Tip the potatoes onto a lined baking tray and bake for 40 minutes until they are golden and crispy on the outside and tender within.: When they roast you will see edges darken to a rich golden brown and hear a casual crackle as moisture evaporates and sugars caramelize. The aroma will turn savory and herbaceous, and if you gently press a piece with a fork it should yield easily, signaling tenderness. The why is that prolonged dry heat creates Maillard reaction on the increased surface area, producing the sought after crunch. A common pitfall is baking at too low a temperature which results in pale, limp skins, so make sure that oven is hot and the tray has room for air circulation.
Sprinkle with the chopped fresh rosemary and sea salt flakes.: The final sprinkle brings a bright herbal lift and bursts of texture from the flakes, and you will immediately notice a fresher, greener scent compared to the roasted herbs. This finishing step adds complexity and a pleasing contrast between crunchy highs and soft interior. One thing to avoid is adding delicate fresh herbs before roasting, which causes them to scorch and lose brightness; reserve them for the finish to preserve their fragrance.