Preheat oven to 170C.

Roast the walnuts in the preheated oven for 5 minutes.

Place butter in an oven-safe dish and allow it to melt in the residual heat of the oven.

Combine cake flour and baking powder in a bowl and whisk.

Crush the roasted walnuts with a rolling pin.

Preheat oven to 180C.

In a separate bowl, whisk the melted butter until it resembles the consistency of mayonnaise.

Add sugar to the butter in two batches, mixing well after each addition.

Stir in the beaten eggs a little at a time, ensuring thorough incorporation.

Add the yuzu jam and crushed walnuts to the dry ingredients and fold briskly until well incorporated.

Pour the batter into muffin molds.

Drop the mold at a height onto a hard surface to release any air bubbles.

Bake in the preheated oven (170C) for about 30 minutes.

Let cool before serving.