In a saucepan, combine flour, granulated sugar, vanilla sugar, egg yolks, and milk. Mix thoroughly. Heat over medium until thickened, stirring constantly. Transfer the mixture to a bowl, cover with plastic wrap touching the surface, and let it cool.

In a separate bowl, beat butter at room temperature until fluffy. Gradually add powdered sugar and continue beating. Then, add the cooled custard mixture and mix well. Finally, add the sweetened condensed milk and blend again. Chill in the refrigerator for about 20 minutes before assembling.

Remove the puff pastry from the freezer and let it sit at room temperature for 20 to 30 minutes. Cut each sheet into 6 pieces without rolling out the dough, and bake at 200°C (400°F) until golden brown, about 35 minutes.

Reduce the temperature to 100°C (212°F). Remove the sheets from the oven, cut each rectangle in half, and return to the oven for another 10-15 minutes. Allow them to cool completely.

Crumble approximately 6 halves (3 full rectangles) of the puff pastry to use as a topping. Take a pastry half, spread the cream, cover with another half, and coat the entire pastry with more cream.

Roll the assembled pastry in the crumbs you prepared and serve. This recipe yields about 9 Mini Napoleons.