Grease/butter a 18cm springform pan. Line the bottom with a circle of parchment paper. Preheat oven to 175C.

In a large mixing bowl, combine 4 egg yolks and 100 g sugar. Beat with an electric hand mixer on high speed 3 minutes or until thick, creamy and lightened in color. Add 150 g almond flour and 1 Tbsp lemon zest and use a spatula to fold and stir together until incorporated. Wash and dry attachments from the mixer.

In a second large mixing bowl, add 4 egg whites and beat together until stiff peaks form (about 2 minutes). Fold the egg whites 1 heaping spoonful at a time into the almond batter, folding with each addition just until incorporated and scraping from the bottom of the bowl to ensure batter has an even consistency.

Once all the egg whites are incorporated and batter is even in consistency (do not over-mix), transfer batter to the prepared baking pan and sprinkle the top with sliced almonds. Bake at 175C for 20 minutes or until top is firm.

Let cake rest in the pan 15 minutes then run a thin spatula around the sides and invert it onto your hand then place it right side up on the rack. Let cool to room temperature and garnish with powdered sugar and lemon slices if desired.