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Pre-heat the oven to 350 degrees Fahrenheit/ 175 degrees Celsius/ Gas Mark 4 and position the rack to the lower third of the oven.
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Prepare an 8" (20.5cm) by 3" (7.5cm) cake pan or an 8" springform pan and line the bottom of the pan with parchment paper.
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Make the clarified butter: heat the butter in a saucepan until hot and bubbly. Continue to cook until the foam subsides. Turn off heat and pour the butter through a fine mesh strainer lined with cheese cloth into a small, 4-5 cup capacity, microwave safe bowl and set aside.
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Sift then measure the oat flour. Place the oat flour into a medium bowl and add 2 tablespoons of the sugar. Thoroughly whisk them together and to remove any clumps.
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In the bowl of a stand mixer, add the remaining sugar, eggs, and salt. Using the whisk attachment of your mixer, whip the egg and sugar on high speed until the batter is light and fluffy. Depending on your mixer it could take about 4-5 minutes, longer if you are using a handheld mixer. 2 visual clues that the batter is ready: the batter will be very fluffy and a light yellow. Also, the volume will have tripled in size, and distinctive well defined streak marks from the whisk attachment will be visible.
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Right before the egg/sugar mixture is finished being whipped, heat the butter in the microwave until hot, careful to prevent the butter from bubbling.
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Remove the bowl with the eggs and sugar from the mixer and sift the oat flour into the bowl in three increments. Gently fold the batter between each addition, careful not to deflate the batter. Once the flour is barely folded into the mixture add a quarter of the batter into the bowl with the butter. Fold the mixture until the butter is thoroughly blended into the batter.
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Add the butter and batter mixture into the remaining batter and gently fold until just blended.
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Pour the batter into the prepared cake pan and bake for 30 to 35 minutes and golden brown on top. A toothpick inserted in the center will come out dry and clean.
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Put the cake pan on a cooling rack and run a knife around the edge of the pan to loosen the cake from the sides.
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Allow the cake to cool slightly in the pan. Invert the cake out of the pan and peel off the parchment paper. Turn the cake right side up, and put the cake back on the cooling rack. Leave alone to completely cool.
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You can bake the cake in advance of preparing the whole cake with frosting and fruit. Once the cake is cool, keep the cake airtight, wrapped in plastic wrap.