Preheat oven to 325°F (170°C). Line a 9x5-inch loaf pan with parchment paper, letting excess extend over all sides of pan.

In bottom of prepared pan, sprinkle demerara sugar.

In a heatproof bowl, combine eggs and granulated sugar. Place bowl over a saucepan of simmering water. Cook, whisking constantly, until an instant-read thermometer registers 110°F (43°C) to 120°F (49°C). Carefully pour mixture into the bowl of a stand mixer fitted with the whisk attachment; beat at medium speed until pale, thick, and ribbon-consistency, 3 to 5 minutes.

In a small bowl, stir together corn syrup and 1 tablespoon (15 grams) hot water until combined and fluid. Add corn syrup mixture to egg mixture, and beat at medium speed until combined. Sift flour, cocoa, and salt onto egg mixture, and gently fold with a balloon whisk until combined. Pour batter into prepared pan. Run a wooden skewer through batter, in vertical and horizontal lines, to release any large air bubbles, being careful not to disturb sugar on bottom.

Bake for 2 minutes. Open oven, and carefully slide oven rack partially out. Lightly mist top of batter with water (2 to 3 sprays). Drag a wooden skewer in a zigzag pattern horizontally and vertically through batter, releasing any large air bubbles that have formed and avoiding sugar on bottom. (Do this quickly and safely so you don’t lose too much heat from the oven or burn yourself.) Slide oven rack back into oven, and repeat baking, spraying, and stirring procedure two more times.

Bake until a wooden pick inserted in center comes out clean and an instant-read thermometer inserted in center registers at least 205°F (96°C), about 45 minutes. Let cool in pan for 5 minutes. Using excess parchment as handles, carefully remove from pan, and place, sugar side up, on a silicone mat or a lightly greased piece of parchment. Let cool for 10 minutes. Wrap warm cake in excess parchment, and then wrap in plastic wrap. Refrigerate, sugar side up, for at least 4 hours, preferably overnight.

When ready to serve, carefully remove plastic wrap and parchment. Using a serrated knife, trim off long sides of cake. Cut crosswise into 1-inch-thick slices. (See Notes.) Store in an airtight container for up to 3 days.