Make the Orange Jam:

In a saucepan over medium-high heat, combine orange pieces, sugar, and water. Boil until the orange is soft.

Mix 1 tbsp of water with cornstarch, then stir into the orange mixture. Boil until thickened like jam.

Transfer to a bowl and let cool completely.

Prepare the Custard Cream:

In a saucepan, whisk together sugar, cornstarch, and salt. Gradually add milk and whisk until smooth.

Heat over medium heat, stirring continuously, until thickened.

In a small bowl, lightly beat egg yolks, then slowly whisk in about ¼ cup of the hot milk mixture.

Return the egg mixture to the saucepan and cook for another 2 minutes until thick.

Remove from heat, add butter and vanilla, and whisk until smooth.

Cover with plastic wrap directly on the surface and refrigerate until fully cooled.

Bake the Cake:

Preheat oven to 338°F (170°C).

In a mixing bowl, beat egg whites with cream of tartar, gradually adding sugar in three batches until stiff peaks form.

In a separate bowl, beat egg yolks and sugar until pale and fluffy.

Gently fold half of the egg white mixture into the yolk mixture, then fold in the remaining egg whites.

In another bowl, mix milk and oil, then blend in 2 tbsp of the egg mixture. Add this back into the main batter along with vanilla.

Sift in flour, baking powder, and salt, folding gently until fully combined.

Pour batter into a 22 cm x 22 cm rectangular tin and bake for 20 minutes. Let cool completely.

Assemble the Cake:

Beat cold heavy cream until soft peaks form, then fold in the custard cream.

Slice the cake in half. Spread custard cream and orange jam over one layer.

Place the second cake layer on top and spread custard cream over it. Top with orange slices, pressing slightly with a scraper.

Smooth the top and sides of the cake with a thin layer of cream. Refrigerate for 3 hours.

Trim the edges and cut into 10 pieces.

Pipe whipped cream on top of each piece and garnish with fresh orange slices.