Preheat oven to 350°F. Butter a 9-inch round springform pan. Coat with sugar, line with parchment paper. Set aside.
In the bowl of a stand mixer fitted with a whisk attachment, combine eggs, sugar, and vanilla. Mix on medium speed for 10 minutes until mixture is very pale in color. Mixture should have thickened. Fold in orange zest.
In a separate bowl, whisk together flour, baking powder, salt, and almond flour. Set aside.
Add dry mixture in three additions, alternating with liquids in 2 additions. Begin and end with dry mixture.*Add 1/3 of flour mixture. Mix until almost combined. Add in melted butter and mix until distributed. Add another 1/3 of flour, follow with shrubbs. Then, add remaining third of flour mixture. Mix until just combined and there are no longer dry streaks of flour. Scrape down sides and bottom of bowl to ensure thorough mixing. Pour batter into prepared springform pan. Spread into an even layer. Sprinkle top of batter with sliced almonds.
Bake cake in center rack of oven. Bake for 28-30 minutes, until the sliced almonds are golden and a tester inserted in center of cake come out clean.
Prepare soaking syrup while the cake is baking.
Use a toothpick or skewer to poke holes all over the cake. Pour ½ cup of syrup over the hot cake. Let sit for 30 minutes to allow cake to absorb liquid. Then, add another ¼ cup syrup over cake. (You will have leftover syrup.)*For best flavor, let cake sit overnight. More details in notes sections. Run a mini offset spatula or thin knife around the edge of the cake to loosen from pan. Unlock springform lock and remove ring from pan. Place cake on serving plate.
For best results, use a serrated knife to slice cake into portions. If desired, drizzle leftover syrup over cake slices. Or, garnish cake with sifted powdered sugar.