Melt 90 g unsalted butter, then continue heating it until the milk solids turn golden brown and smell nutty. Set the browned butter aside to cool until lukewarm.

Whisk 2 eggs with 90 g granulated sugar until the mixture becomes pale, thick, and airy.

Whisk 5 ml vanilla extract and 10 g Biscoff spread into the egg mixture until smooth.

Sift 100 g all-purpose flour and 3 g baking powder into the bowl. Fold just until no dry streaks remain.

Whisk the cooled browned butter and 10 ml milk into the batter until combined.

Crush 7 to 8 Biscoff biscuits into small crumbs. Fold the crumbs evenly through the batter.

Transfer the batter to an airtight container. Refrigerate it for at least 12 hours until thoroughly chilled.

Preheat the oven to 190°C (375°F). Brush a madeleine pan with 5 g melted butter and dust it with 5 g flour.

Fill each prepared madeleine mold about three-quarters full with cold batter. Use a piping bag for neat, even portions.

Bake the madeleines at 190°C (375°F) for 12 to 14 minutes until risen, golden at the edges, and springy.

Remove the madeleines from the pan and cool briefly on a cooling rack. Serve warm for a crisp exterior and soft center.