Take butter and eggs out of refrigerator to come to room temperature.
Prep
Measure and prepare all of the ingredients.
Butter and flour* the sides of the cake tin, and line the bottom with a round of parchment paper. Set the oven rack in the lower third of the oven and preheat the oven to 350°F (180°C).
Make the Cake
Beat the butter and sugar together with an electric mixer at medium speed, or a whisk, until pale yellow and fluffy. Add the egg yolks and beat until blended.
Heat the chocolate and coffee together over low heat in a bain-marie setup using a double boiler or a large heatproof bowl/pan set over a saucepan of shallow water, stirring occasionally with a wooden spoon until the chocolate is just melted. Immediately take off heat and stir again until smooth. Stir the melted chocolate into the butter mixture, followed by the almond extract.
In a separate bowl, beat the egg whites to soft peaks at medium-high speed. Then gradually sprinkle on 1 Tbspsugar to help stabilize the whites, and beat at high speed to stiff peaks.
With a rubber spatula, alternate folding into the mixture a third of the whipped egg whites at a time with the almond flour and flour until all is added. Pour into the prepared cake tin and smooth the batter out evenly.
Bake
Bake for around 18 to 23 minutes, or untilpuffed, yet still looks undercooked in the center. I find a good rule of thumb is to take it out of the oven when (looking closely) only 1 inch (2.5cm) of the border around the edge looks dry and set. A toothpick test comes out almost dry on the border and still oily in the center.Take care not to overcook it! You want to intentionally undercook the cake, as it will lose its moist creaminess if cooked all the way through.
Cool for 10 minutes. Then slide a knife around the edges of the pan and invert the cake upside down onto a cake rack to cool for at least an hour before frosting. Or enjoy unfrosted (see notes). Serve at room temperature.
Notes
Flouring the Cake Pan
If you don't plan to frost the cake, keep the sides of the cake dark for serving by dusting with cocoa powder instead of flour. Either way, you don't need to butter and flour the bottom of the pan when using parchment paper.
Melting Chocolate in Microwave
Note: If you prefer to melt the chocolate in the microwave, whisk the sugar only with the egg yolks in step 3, and melt the butter together with the chocolate, followed by any liquid flavoring.
To melt the chocolate in the microwave: roughly chop or break into small even-sized pieces and cook at 50% power for 1 minute, then take out and stir, repeating until almost completely melted. Then stir again and let continue to melt on the counter.
Decorating
Without Frosting - Sprinkle a little powdered sugar or cocoa around the border.
With Chocolate Curls - Sprinkle over the frosting, either on top, around the sides, or over the entire cake. I often make quick curls using a vegetable peeler.
With Sliced Almonds - You can lightly dry-roast sliced almonds in a skillet and place them at the bottom of the cake tin before pouring the batter in. Or you can sprinkle them on the frosting, either on top or just on the sides. One way to do this is by holding a long fat spatula topped with nuts in one hand and leaning the sides of the frosted cake, supported, down against the nuts with your other hand.
Serving & Storing
Stores well, covered, at room temperature for up to 2 days, and in the refrigerator for up to a week. Let come to room temperature for serving.
Freezes beautifully. I like to freeze it in individual portions. Cover tightly. Tip: If frosted, store in a container with high sides (2”, 5cm) so the cover doesn't stick to the icing. Take out of the freezer at least 2½ hours before serving to let thaw on the counter. Or let thaw in the refrigerator overnight and take out of refrigerator at least 1 hour before serving.
Originally adapted from Reine de Saba recipes of Simone Beck and Julia Child.