Prepare the craquelin: mix softened butter, brown sugar and flour until it forms a dough. Roll between parchment sheets to ~2 mm thickness, then refrigerate or freeze it while preparing the choux dough. :contentReference[oaicite:1]{index=1}

Preheat oven to 190 °C (≈ 375 °F). Line a baking sheet with parchment paper.

In a saucepan, combine water (or water + milk), butter, salt (and sugar-if using) and bring to a boil. Then remove from heat and add all the flour at once. Stir vigorously until the dough forms a ball, then let it cool slightly. :contentReference[oaicite:2]{index=2}

Beat in the eggs one at a time (or gradually) until dough is smooth, shiny and pipeable.

Transfer the dough to a piping bag (or use two spoons) and pipe mounds on the prepared baking sheet. Cut small discs from the chilled craquelin dough and place one on top of each choux mound. :contentReference[oaicite:3]{index=3}

Bake for about 25–30 minutes until the puffs are puffed, golden and crisp. Then prick each with a skewer to release steam and let cool on a wire rack. :contentReference[oaicite:4]{index=4}

Meanwhile, make the coffee pastry cream: in a bowl whisk egg yolks, sugar, and cornstarch until smooth. In a saucepan, heat milk with instant coffee (and vanilla if using) until near boiling. Gradually temper the hot milk into the yolk mixture, stirring constantly. Return to medium heat and cook until the mixture thickens into a custard. Let cool, then fold in whipped cream for a lighter filling. :contentReference[oaicite:5]{index=5}

Once the choux puffs are completely cooled, poke a small hole at the bottom or side of each puff, and pipe in the coffee cream until filled.

Optionally dust with powdered sugar or drizzle a little chocolate or caramel on top before serving. Best enjoyed fresh for the crisp-shell & creamy interior contrast.