Heat the oven to 375° F. (non-convection/not fan-assisted).
Line the bottom of an 8-inch springform pan with parchment paper and spray the parchment and sides of the pan with cooking or baking spray.
In a medium bowl, whisk together the melted butter, sugar, cocoa powder, vanilla, and salt until well mixed. Add the eggs one at a time and whisk until smooth. Fold the flour into the batter just until it is combined, then scrape the batter into the prepared pan, smoothing the top.
Tip! The precise baking time to get the perfect consistency of this cake will vary from pan to pan (darker pans will cook more quickly) and from oven to oven. The toothpick test for doneness is helpful, but we aren't looking for the toothpick to come out clean. Instead, test in the very centre, where it should be quite wet, then test about halfway out from the centre, where it should be less wet and more set. If the second test is as wet as the one from the centre, let it bake another minute or two.
Bake until the cake forms a thin skin on top, but the batter is still loose in the centre, about 13-15 minutes (it is typically 14 minutes in my oven). Let it stand for a few minutes, then run a knife around the outside and loosen and remove the ring. Slice and enjoy. Leftovers can be refrigerated.
Notes
Note 1:You can make your own cake flour by measuring out 1 cup of all-purpose flour, removing 2 Tablespoons of it and replacing it with 2 Tablespoons of cornstarch. Whisk together and use in this recipe. In a pinch, all-purpose flour can be used.