In a shallow bowl, whisk together the egg and 50mL of milk until well combined.
In a saucepan, melt the butter over medium-low heat.
Dip each bread slice into the egg mixture, letting it soak for 10-15 seconds on each side.
Place the soaked bread slices in the hot pan and fry until golden brown on one side. Flip and cook the other side until golden brown as well.
In a small pot, combine 100mL of milk and cornstarch. Bring to a boil over medium heat, stirring constantly until the mixture begins to thicken. Remove from heat immediately, then stir in 1 tablespoon of condensed milk until fully combined.
In a small bowl, mix together the Ovaltine powder and kinako powder.
Place one slice of the cooked French toast on a plate and spread a thin layer of condensed milk on top.
Place the second slice of French toast on top of the first. Use a spoon to gently press down the center of the top slice, creating a small indentation or "hole."
Pour the thickened milk mixture into the indentation on the toast.
Sift the Ovaltine and kinako powder mixture evenly over the toast, then finish by sifting a light layer of cocoa powder on top.