Step 1: For the bone broth reduction, pour 250ml of high-quality beef bone broth into a small saucepan. Bring to a rapid boil over high heat, then immediately reduce heat to a vigorous simmer. Allow to reduce by half, stirring occasionally, for 10-12 minutes until it coats the back of a spoon and has a concentrated flavor.

Step 2: While the broth reduces, prepare the bone marrow. If using bone marrow bones, scoop out 25g of marrow. If it's solid, gently warm it in a small, heatproof bowl set over a pot of simmering water (bain-marie) until just softened, about 2-3 minutes. Do not melt completely.

Step 3: Transfer the softened bone marrow to a small, chilled bowl. Using a mini whisk or a fork, vigorously whisk the marrow for 2-3 minutes until it becomes light, airy, and pale in color. Season with 0.5g salt. It should have a texture similar to soft butter.

Step 4: Once the bone broth reduction is complete, pour it into a small serving ramekin or cup. Carefully spoon the whipped bone marrow on top of the hot broth reduction. Serve immediately, allowing the marrow to slightly melt into the rich broth as you enjoy.