In a medium bowl, mix 2 tablespoons of softened unsalted butter, 2 tablespoons of light brown sugar, and 3 tablespoons of all-purpose flour until combined into a dough. Roll it out between two sheets of parchment paper to about ⅛ inch thick, then freeze for at least 30 minutes until firm.

In a medium saucepan, combine 1 cup of water (or half water and half milk), ½ cup of unsalted butter, and a pinch of salt over medium heat. Bring to a boil, then remove from heat and stir in 1 cup of sifted all-purpose flour until a cohesive ball forms. Let it cool for about 5 minutes, then add 4 room-temperature eggs one at a time, mixing well between additions until the dough is smooth and shiny.

Transfer the smooth choux pastry into a piping bag fitted with a large round tip. On a parchment-lined baking sheet, pipe mounds about 1.5 inches wide, leaving space between each. Remove the craquelin from the freezer, cut it into discs that match the size of your pastry mounds, and place one on top of each mound.

Preheat your oven to 400°F (200°C). Place the baking sheet in the oven and bake for 10 minutes; do not open the oven door. After 10 minutes, lower the oven temperature to 350°F (175°C) and bake for an additional 15 to 20 minutes, until they are golden brown and dry to the touch.

In a saucepan, heat 2 cups of whole milk with 2 tablespoons of instant coffee until just warm. In another bowl, whisk together 4 egg yolks, ½ cup of granulated sugar, and 2 tablespoons of cornstarch until smooth. Gradually combine the warm milk into the egg mixture, then return to medium heat. Stir constantly until it thickens, just around 5 minutes, and then let it cool to room temperature.

Once the coffee custard has cooled, gently fold in 1 cup of whipped heavy cream to lighten the mixture. Cover the bowl with plastic wrap and chill in the refrigerator while you prepare the puffs for filling.

After the choux puffs have completely cooled, use a small sharp knife to poke a hole in the bottom of each puff. Transfer the chilled coffee custard into a piping bag with a small round tip and carefully fill each puff until they feel slightly heavy.

Serve your completed Crispy Coffee Cream Choux Puffs chilled, either plain or dusted with powdered sugar for an elegant touch.