Preheat oven to 350F. Grease an 8 or 9 inch springform pan and line the bottom with parchment paper
In a large bowl, add the sugar and the egg yolks (save the egg whites in a separate bowl). Beat with a hand mixer (or by hand) until light and foamy
Add the orange zest, orange juice, oil, and almond extract (or orange extract) if using. Beat for two more minutes
Add the almond flour and baking powder. Mix until combined and smooth
Beat the egg whites in a separate bowl until firm. Fold into the cake batter
Pour into the prepared pan, level, and sprinkle with almond slices
Bake for about 40 minutes or until golden and set. Test with a toothpick for readiness. Let cool, remove from the pan and serve with powdered sugar