Separate the eggs into whites and yolks.
Add the sweetener to the egg whites in three portions, whipping each time until stiff peaks form.
Add sweetener to the yolks and whisk until pale.
Mix vegetable oil and milk into the yolk mixture.
Gently fold whipped egg whites into the yolk batter in three additions.
Sift cake flour in and mix lightly.
Pour the batter into silicone cups.
Bake in a preheated oven at 170°C (338°F) for 20–25 minutes.
Whip fresh cream with sweetener and top the finished castella.