Make the churro dough: in a medium saucepan combine Water, a pinch of Salt, a tablespoon or two of Granulated Sugar and a couple of tablespoons of Olive Oil. Bring to a brisk simmer over medium heat.
Remove the pan from the heat and add all of the All-Purpose Flour at once, stirring vigorously with a wooden spoon until the mixture pulls away from the sides of the pan and forms a smooth, slightly sticky dough.
Allow the dough to cool for a few minutes so it stops steaming, then transfer it to a piping bag fitted with a large star tip (or a sturdy zip-top bag with the corner snipped).
Heat a deep, heavy-bottomed pot with enough Sunflower Oil to submerge the churros (about 2–3 inches) to 350°F (175°C) or medium-high heat — a small piece of dough should sizzle and rise to the surface immediately.
Pipe 3–5 inch strips of dough directly into the hot oil, cutting them with scissors or a knife as you go; fry in batches so they have room to puff. Turn once and fry until evenly golden and crisp, about 2–4 minutes per batch.
Drain the cooked churros on paper towels and immediately toss them in additional granulated sugar while still warm so the sugar adheres and melts slightly into the ridges.
Make the chocolate dipping sauce: whisk a small spoonful of Cornstarch into a couple tablespoons of Whole Milk to make a smooth slurry. In a small saucepan, heat the remaining Whole Milk with a small spoonful of Granulated Sugar until just simmering.
Remove the milk from the heat, add chopped Dark Chocolate, and stir until melted and smooth. Return to low heat and whisk in the cornstarch slurry, cooking gently until the sauce thickens to a dip-able consistency. Keep warm.
Serve the churros hot with the glossy dark chocolate sauce for dipping. Enjoy immediately for the best contrast of crisp pastry and warm chocolate.
