Make the Caramel Sauce

1. Add the sugar and water to a saucepan and heat over medium-low heat until the sugar melts. Do not stir the mixture.
2. Once the caramel starts to bubble, gently swirl the pan occasionally until the sugar turns an amber color.
3. Turn off the heat and carefully pour in the double cream. Stir until smooth and well combined.
4. Transfer the caramel sauce to a heatproof bowl and let it cool completely before using.

Make the Cake Batter

1. Preheat the oven to 160°C (325°F). Prepare an unlined 7-inch chiffon cake pan. Do not grease it.
2. In a medium mixing bowl, beat the egg yolks, vegetable oil, milk and vanilla extract at medium speed until fully emulsified.
3. Sift in the cake flour and salt.  Gently mix at medium speed until just combined.  Switch to a silicon spatula and finish mixing everything by hand until well mixed.

Make the Meringue

1. In a clean, grease-free, bowl add the lemon juice  into the egg whites and begin whisking until foamy.
2. Gradually add 50 g sugar, one-third at a time, giving each addition 30 seconds to a minute to let the sugar dissolve.
3. Once all the sugar is added, turn the mixer to medium high speed and continue to whisk until stiff peaks form.

Combine the Mixtures

1. Fold a small amount of the meringue into the egg yolk batter to lighten the mixture.
2. Gradually add the remaining meringue in small batches, gently folding after each addition until no white streaks remain before adding the next batch. 
3. Scoop a small amount of the batter into the cooled caramel sauce and mix until smooth.
4. Pour the caramel mixture back into the remaining batter and gently fold a few times with a spatula. Avoid fully mixing the batter—the caramel should remain lightly streaked to preserve the airy texture.
5. Pour the batter into an ungreased chiffon tube pan, preferably in one spot, allowing it to spread naturally.
6. Lift the pan a few inches above the counter and drop it a few times to release any large air bubbles.
7. Run a skewer through the batter to pop any remaining small air pockets, then gently shake the pan to level and smooth the surface before baking.

Bake and cool the Cake

1. Bake in a preheated oven at 160°C for 35 – 38 minutes, or when an inserted skewer comes out clean.
2. Immediately invert the cake pan and let it cool completely upside down to prevent collapse.
3. Once cooled, gently run around the edges and the funnel of the pan with a spatula to release the cake.
4. Gently run around the base of the pan with the spatula again to release the cake completely.
5. Prepare your serving plate or cake stand, then flip the tube pan upside down.
6. The cake should come out cleanly and beautifully.

Make the Caramel Whipped Cream (Optional)

1. Chill a mixing bowl, preferably stainless steel, in the refrigerator while you prepare the caramel sauce.
2. Add the sugar and water to a saucepan over medium-low heat. Allow the sugar to dissolve completely without stirring.
3. Once the syrup begins to bubble, gently swirl the pan occasionally until it turns a deep amber colour.
4. Remove from the heat and carefully pour in 100 ml chilled double cream. Stir until the mixture is smooth and fully combined.
5. Add the butter and salt, stirring until the butter has fully melted and the salt has dissolved.
6. Transfer the caramel sauce to a heatproof bowl and let it cool completely at room temperature. Once cooled, transfer it to a piping bag for easier handling and application.
7. In the chilled bowl, add the remaining 200 ml chilled double cream, icing sugar, and vanilla extract. Whisk on medium speed until soft peaks form.
8. Add a generous amount of the cooled caramel sauce into the whipped cream and gently fold it through to create a marbled effect. Avoid over mixing to keep visible swirls of caramel.
9. Cover and refrigerate until ready to use.
   

Decorate the Cake (Optional)

1. Toast the almond flakes in a cast iron pan over medium-low heat, stirring constantly to prevent burning, until they turn golden and release a nutty aroma.
2. Transfer to foil or a tray and allow them to cool completely before using.
3. Apply a thin layer of caramel whipped cream to crumb coat the cake.
4. Cover the sides of the cake, including the funnel, with the toasted almond flakes.
5. Pipe your desired design on top using the caramel whipped cream.
6. Finish with the remaining caramel sauce for garnish.
7. Slice and serve — enjoy your beautifully decorated caramel chiffon cake!