Make the Custard: In a saucepan, whisk together the egg, sugar (regular and vanilla), cornstarch, and milk. Heat the mixture over medium-high heat, stirring constantly, until it thickens and begins to bubble. This will take about 5-7 minutes. Remove from heat and let cool to room temperature.

Preheat the Oven: Preheat your oven to 170°C (338°F). Line a 27 x 18 cm baking dish with baking paper for easy cleanup.

Prepare the Cake Batter: In a large bowl, cream together the eggs, sugar, and salt with an electric mixer (or by hand) until light and fluffy. This will take about 3-4 minutes.

Combine Wet and Dry Ingredients: Whisk in the milk and vegetable oil to the creamed mixture. In a separate bowl, whisk together the flour and baking powder. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Don't overmix!

Gently Fold in Almonds: Gently fold in the ground almonds using a rubber spatula.

Assemble and Bake: Pour half of the cake batter into the prepared baking dish. Spread the cooled custard evenly over the batter. Top with the remaining cake batter. Sprinkle the sliced almonds over the top of the cake.

Bake the Cake: Bake the cake in the preheated oven for 40 minutes, or until a toothpick inserted into the center comes out clean.

Serve: Let the cake cool slightly in the pan before transferring it to a wire rack to cool completely. Dust the top with powdered sugar before serving.