Set aside 6 ladyfinger biscuits and crush them in a food processor until finely powdered. Dip the remaining ladyfingers briefly in a little fresh milk and arrange them in a square or rectangular ovenproof dish.
Prepare the Custard
Place the milk and cream in a saucepan over medium-high heat. Add half of the sugar without stirring. This helps prevent the milk from scorching.
In a heatproof bowl, whisk the eggs together with the remaining sugar and cornstarch.
Once the milk mixture begins to steam vigorously (just before it reaches boiling point), stir to dissolve the sugar. Gradually pour the hot milk mixture into the bowl with the eggs in two additions, whisking continuously to combine well.
Strain the mixture back into the saucepan. This ensures a smooth, velvety custard without any bits of eggshell or undissolved egg. Add the vanilla.
Stir the custard continuously over medium-low heat until it begins to boil. Once it comes to a boil, continue stirring for another minute. Remove from the heat and add the butter. Stir until the butter has completely melted and is fully incorporated.
If the dish you are using to serve the dessert is heatproof (such as a Pyrex dish), you can immediately pour the custard over the ladyfingers and cover with plastic wrap, pressing it directly onto the surface of the custard.
If your serving dish is not heatproof, it is better to let the custard cool slightly, stirring occasionally to prevent a skin from forming. Then transfer it to the serving dish, over the ladyfingers, and cover with plastic wrap until completely cooled.
Dissolve the strawberry jelly and allow it to cool without setting. To speed up the cooling process, you can dissolve the jelly in ½ cup boiling water and, once dissolved, add ½ cup cold water.
Place the strawberries in a food processor and pulse on low speed until they are roughly chopped. Be careful not to purée them completely. Transfer them to a bowl and mix with the 3 tablespoons of sugar and the Grand Marnier, if using.
Spread the strawberries over the cooled custard. Using a large spoon, gently pour the strawberry jelly over the strawberries. Sprinkle with the crushed ladyfingers and refrigerate until the jelly has completely set.
Whip the heavy cream with the icing sugar and vanilla, then spread it over the strawberry jelly layer.
Prepare the dessert the day before serving.