Bring a medium pot of water to a boil. Add the whole apricots and blanche the fruit for one minute. Use a pair of tongs or a slotted spoon to transfer the whole fruit to the ice bath. Shock the fruit for one minute, then remove from the water and peel the skins from the apricots. They should come off easily.
Halve the apricots and thinly slice. Set aside.
PREPARE THE ALMOND TOPPING:
In a small bowl whisk the egg white until light and frothy. Add the sliced almonds and stir to coat. Place a fine mesh sieve over a bowl and pour the almond-egg mixture into the sieve. Spread the almonds out to open up more surface area. Set aside for the excess egg to drip away.
FOR THE CAKE:
Preheat the oven to 350°. Spray a 9" springform pan with nonstick spray. Line the bottom of the pan with parchment paper and spray again. Set aside.
MAKE THE CAKE BATTER:
In a small bowl, whisk together the flour, baking powder, salt and vanilla powder. Set aside.
In a large bowl, combine the sugar and butter. Beat on high with a hand mixer or stand mixer for 3-5 minutes until light and fluffy. Add the eggs one at a time, beating until they are completely mixed into the butter mixture before adding the next egg. Finish with the last egg and almond extract, and beat just until combined while scraping the sides of the bowl with a spatula.
In a small bowl or glass measuring cup, whisk together the sour cream and buttermilk until smooth.
Beat in 1/3 of the dry ingredients to the butter mixture until just combined, then beat in 1/2 of the sour cream mixture. Continue alternating wet and dry ingredients finishing with the flour.
Add the sliced apricots to the batter and fold together. Transfer the cake batter to the prepared pan and smooth the top with a spatula.
The egg white almonds should feel tacky, not wet. If they're very wet, dab them with a paper towel. Sprinkle the almonds evenly over the top of the cake batter. Sprinkle the sugar over evenly over the almonds.
Set the cake in the center of the oven and bake for 40-45 minutes or until the top is lightly golden and a cake tester comes out clean. Let the cake rest for 10 minutes, then run a sharp knife around the edge of the pan. Release the collar of the spring form pan and transfer the cake to a cooling rack to cool completely.
When cake has cooled, place it on a cake platter and serve with whipped cream or ice cream if desired.