Center a rack in the oven and preheat to 350 degrees F. Butter a 9-inch springform pan, line the bottom with parchment paper, butter the paper, dust the inside of the pan with flour and tap out the excess. Place pan on a baking sheet.
Separate the eggs, putting the whites in a large mixing bowl and the yolks in a small bowl.
To the top of a double boiler (or a small heatproof bowl placed above simmering water), add the chocolate, sugar, butter, and coffee. Stir occasionally until the chocolate and butter are melted; the sugar may still be grainy - that is fine.
Transfer the mixture to the counter and let sit for 3 minutes to cool. Using a rubber spatula, stir in the yolks one by one, then fold in the flour.
Working with the whisk attachment of a hand- or stand-mixer, beat the egg whites with a pinch of salt until they hold firm but glossy peaks. Using the spatula, stir about one quarter of the beaten egg whites into the batter, then gently fold in the rest.
Scrape the batter into the prepared pan and jiggle the pan from side to side a couple of times to even the batter.
Bake for 35-45 minutes, or until the cake has risen evenly (it might rise around the edges and you'll think it's done, but give it a few more minutes, and the center will puff too) and the top has firmed (it will probably be cracked) and doesn't shimmy when tapped; a thin knife inserted into the center should come out just slightly streaked with chocolate. Transfer pan to a cooling rack and let cake rest for 5 to 10 minutes.
Run a blunt knife gently around the edges of the cake and remove the sides of the pan. Carefully turn the cake over onto a cooling rack and remove the pan bottom and the parchment paper. Invert the cake onto another cooling rack and cool to room temperature right side up. As the cake cools it may sink.
To make the glaze, turn the cooled cake onto another cooling rack so you will be glazing the flat bottom, and place the rack over a baking sheet lined with parchment or wax paper to catch any drips.
Put the chocolate in the top of a double boiler and melt above simmering water (or use microwave to melt). The chocolate should be just melted and only warm, not hot. Meanwhile, bring the cream to a boil in a small saucepan. Pour the hot cream over the chocolate and stir very gently with a rubber spatula until the mixture is smooth and shiny. Stir in the corn syrup.
Pour the glaze over the cake and smooth the top with a long metal icing spatula. Don't worry if the glaze drips unevenly down the sides of the cake, it will add to its charm. Decorate if desired. Allow the glaze to set at room temperature, or slip the cake into the refrigerator about 20 minutes.