Combine the butter, sugar, milk, egg yolks, and salt in a heatproof bowl set over a saucepan (2-quart / 2L) of gently simmering water, making sure the bowl doesn't touch the water.

Whisk constantly over medium-low heat for 8 to 10 minutes, until the mixture thickens enough to coat the back of a spoon and reaches 71 C / 160 F. Do not let it boil or the yolks will scramble.

Remove the bowl from the heat and whisk in the cocoa powder until smooth.

Fold in the ground walnuts and vanilla extract until you have a thick, spreadable paste. Let it cool to room temperature, about 20 minutes, it will thicken further as it sits.

Place one wafer sheet (20 cm / 8-inch round) on a flat plate or cake board. Spread with a thin, even layer of filling, about 100 g, all the way to the edges.

Repeat with 3 more wafer sheets and filling, then top with the last plain wafer sheet.

Cover the stack with parchment paper and set something flat and heavy on top, like a cutting board weighed down with a couple of cans. Press gently to compress the layers.

Refrigerate for at least 8 hours, or overnight, so the wafers soften and absorb moisture from the filling.

Trim the edges with a sharp serrated knife if needed, then cut into 12 squares. Wipe the blade between cuts for clean edges.