Heat the oven to 350 degrees. Lightly grease an 8-inch round cake pan with oil.

In a large bowl, whisk together the granulated sugar, cocoa powder, baking powder, baking soda and salt. Add the coffee and whisk to combine and bloom the cocoa (reconstituting it and amplifying its flavor). Add the oil, sour cream and egg and whisk until smooth. Add the flour and whisk until just combined and no dry pockets of flour remain. Scrape the batter evenly into the prepared pan.

Bake until a toothpick inserted into the center comes out clean, 25 to 35 minutes. Set the cake in its pan over a wire rack and let cool completely. (The cake can be baked 1 day in advance of topping; tightly cover the completely cooled cake in its pan if storing overnight.)

In a medium, microwave-safe bowl, microwave the heavy cream until just bubbling around the sides and steam is coming off the surface, about 45 seconds. Add ⅓ cup/105 grams chocolate hazelnut spread and ¼ teaspoon salt to the cream and stir until smooth and combined. Pour over the cooled cake in an even layer, scraping every drop out of the bowl. (Alternatively, the cream can be heated in a small saucepan over medium heat until simmering. Pour the cream over the chocolate hazelnut spread and salt in a medium heatproof bowl.)

To the same, now-empty bowl, microwave the bittersweet chocolate in 20-second increments, stirring in between, until completely melted, about 1 minute. Add the remaining ½ cup/158 grams chocolate hazelnut spread and ¼ teaspoon kosher salt and stir to combine. (Alternatively, the chocolate, chocolate hazelnut spread and salt can be melted together in a double boiler.) Add the cornflakes and stir until the cornflakes are completely coated. Spoon the cornflake mixture evenly over the glazed cake, gently spreading with the back of the spoon into an even layer.

Serve immediately, while the topping is still runny. Leftover cake can be stored loosely covered in the pan at cool room temperature for up to 2 days.