Cream softened butter, powdered sugar, and salt together until smooth. Add egg yolk and vanilla extract, mixing well.

Sift cake flour and almond flour into the mixture. Use a spatula to mix until crumbly.

Separate 60g of the dough into another bowl and mix in cocoa powder until fully incorporated.

Form each dough into a ball. Combine small portions of the cocoa dough with the white dough to create a marble effect.

Shape the dough into a rough cylindrical shape. Place it on parchment paper and fold the paper over. Use a ruler to roll the dough into a smooth cylinder.

Wrap the cylinder in parchment paper and freeze for 1 hour until firm.

Let the dough sit at room temperature briefly to soften slightly. Slice into 0.5 cm thick rounds and place on a baking tray.