Note: Whipped cream with vanilla extract adds richness to the cake but is not absolutely essential. It does make folding-in a bit trickier.

Procedure:

1)  Heat oven to 375. Place eggs, in their shells, in a medium bowl and fill the bowl with water at 100 F (body temperature), allowing eggs to warm while you measure ingredients and set up. Lightly grease the inside of a 6" circular cake pan and line with bakers parchment, making sure parchment along the sides is at least 1 inch taller than cake pan. Do not grease the parchment. Place bowl of stand mixer in a large sauce pan and add enough water to saucepan to reach to within 1 inch of the saucepan rim. Remove mixer bowl and place saucepan over low heat.

2)  Place the 1 additional Tablespoon of sugar, the oat flour, the salt, and the baking powder in the second of the medium bowls. Whisk to combine.

3) In the third medium bowl, whip the cream and vanilla extract, if using, with a hand mixer until soft peaks form, about 3 minutes. 

4)   Measure sugar into bowl of the stand mixer. Add the eggs and the yolk and begin hand-whisking. Place the mixer base into the water simmering on the stove. Continue to hand-whisk eggs and sugar together, occasionally dipping instant read thermometer or a finger into the mix. Stir until 100 F is reached (body temperature) and all the sugar is dissolved into the eggs. Eggs should begin to foam. Lift stand mixer bowl from water, turn off heat, allow mixer bowl to drain, then move to mixer base. Using the whisk attachment, whip eggs on highest setting for 2 minutes, until they have become pale, foamy, and risen. Reduce speed to half (medium) and continue to beat 10 more minutes. Eggs should have tripled in volume, become glossy  and noticeably stiffer, with ridges forming behind the whisk wires. Remove bowl. 

5) Gently fold flour blend into eggs, taking care to minimize egg collapse (there will be some). Remove 1/2 cup batter and whip it into the whipped cream, if using. Gently fold whipped cream into eggs (watch closely - they are the exact same color). Pour batter into prepared cake pan. Bake about 25 minutes, or until an inserted toothpick comes out clean. 

6) Remove cake to a cooling rack. After five minutes, run a knife around the inside of the pan between parchment and pan to prevent sticking. Allow cake to cool an hour, then invert to remove from pan. Place a flat plate on the bottom of the cake and re-invert and allow to finish cooling completely.

7) Meanwhile, make the apricot purée. Slice apricots in half and remove seeds. Cut into small pieces. Place, with sugar and 1/2 cup water, into a small saucepan. Bring to a boil over high heat then turn down to a rolling simmer. When liquid has reduced by half and the fruits are mostly disintegrated, remove from heat. To make a smooth purée, use an immersion blender.

8) Make frosting. Using the hand mixer, add butter, the confectioners sugar and the almond extract. Start mixer on low to blend all ingredients, then increase speed to medium-high. Once frosting begins to fluff, stop and taste. If it needs richness, add heavy cream, 1 teaspoon at a time, while whipping. If it needs tartness, add a few drops of lemon juice. Let stand for 5 minutes then whip on high speed until frosting is very light and fluffy.

9) Slice cake in half axially (to produce two equal discs). Layer the apricot purée across the top of the bottom disc, spreading to about 1/2 inch of the edge. Position the second disc on top of the purée layer, then, using an offset spatula or a pastry bag with decorative tip, frost the top of the cake. You may need to steady the top half with one hand so it does not skid off the purée.

NOTE: To frost the entire top and sides of the cake, double frosting ingredients.