Get Ready: Preheat your oven to 350°F (175°C). Lightly butter a 9-inch round cake pan and line the bottom with parchment paper; lightly butter the parchment. This simple step ensures a perfect release!
Combine Dry Ingredients: In a mixing bowl, sift together the almond flour, cornstarch, and salt. Sifting helps achieve a wonderfully smooth cake texture. Set this aside.
Whip Eggs and Sugar: In a large bowl of a stand mixer fitted with a whisk attachment, whip the warm eggs on medium speed until they become frothy. Add the sugar and increase the mixer speed to high, continuing to whip for 7-10 minutes. The mixture should be very pale and thick, forming ribbons when you lift the whisk. This process incorporates plenty of air for a light, airy cake.
Gently Fold in Almond Mixture: Pour the sifted almond flour mixture into the whipped eggs and sugar. Using a wide rubber spatula, gently fold the ingredients together, scraping the bottom and sides of the bowl, until the flour is just incorporated and no dry streaks remain. Be careful not to deflate the airy egg mixture.
Prepare the Butter: Place the cubed butter in a heat-proof measuring cup. Microwave at 50% power for 10-second intervals, stirring thoroughly after each. Continue until the butter is soft enough to flow like thick ranch dressing, but not fully melted or separated into oil and solids. This ensures smooth emulsification.
Finish the Batter: With the mixer running on low speed, slowly pour the prepared butter into the batter, followed by the Grand Marnier (or your chosen alternative) and almond extract. Mix for just a few seconds until everything is just combined. Give it one or two final gentle folds by hand to ensure everything is perfectly incorporated while preserving the air.
Bake with Care: Pour your delicate batter into the prepared cake pan. For extra flair, sprinkle the top with slivered almonds if you wish. Bake for 40-45 minutes, or until the top and edges are beautifully golden brown, and a toothpick inserted into the center comes out clean, indicating it's perfectly cooked.
Cool & Chill: Remove the cake from the oven and let it cool completely on a wire rack. For the best, most magical texture, refrigerate the cake for 2-3 hours, or even overnight. It tastes even better chilled!
Serve & Enjoy: When you're ready to indulge, dust the top with a sprinkle of powdered sugar, if desired, slice, and serve this delightful, moist cake at room temperature. Store any leftover cake in an airtight container in the refrigerator for up to 5 days – it actually gets moister over time!