In a mixing bowl, combine the softened butter, brown sugar, all-purpose flour, and vanilla extract. Mix until you achieve a smooth dough that resembles a thick paste.

Roll the dough between two sheets of parchment paper to about 1/8 inch thick. Freeze for about 15-20 minutes until firm, making it easier to cut and place on your pastry mounds.

In a saucepan, combine the water, unsalted butter, sugar, and salt. Bring this mixture to a gentle boil over medium heat, stirring occasionally.

Once boiling, remove the saucepan from heat and add the flour all at once. Stir vigorously with a wooden spoon until the mixture forms a ball and pulls away from the sides of the pan.

Allow the mixture to cool slightly for about 5 minutes. Add the eggs one at a time, mixing well after each addition until the dough is smooth and glossy.

Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper to prevent sticking.

Transfer the choux pastry into a piping bag fitted with a round tip. Pipe small mounds, roughly 1.5 inches in diameter, onto the prepared baking sheet, leaving space between each mound.

Remove the frozen craquelin from the freezer and cut it into circles slightly smaller than the choux mounds. Place one circle atop each piped mound, creating a tasty topping.

Bake in the preheated oven for about 25-30 minutes, or until the choux are puffed up and golden brown. Remember to avoid opening the oven door during the baking process.

Once baked, turn off the oven and crack the door open. Let the choux cool in the oven for about 10 minutes before transferring them to a wire rack to cool completely.

In a mixing bowl, whip together the heavy cream, powdered sugar, and vanilla extract until you achieve soft peaks. This will create a wonderful, airy filling.

Once the choux are completely cool, use a small knife to make a hole in the bottom of each. Pipe the whipped cream filling into each choux until they are generously filled.

Arrange the filled choux au craquelin on a serving platter and enjoy this delightful treat with family and friends!