Sift the cocoa powder and combine with the flour, sugar, and salt.

Mix in the room-temperature butter, beaten eggs, and vanilla extract until a smooth, cohesive dough forms (add a little flour if sticky, or a little milk if too dry).

Roll the dough to about 1/8-inch thickness, chilling it first for crisp edges, then cut into rounds.

Preheat the oven to 350°F (175°C) and bake for 8-10 minutes, until slightly darkened at the edges. Let cool completely before filling.

For the filling, sift the powdered sugar and beat with the shortening, vanilla, and a splash of water or milk on low speed until smooth and spreadable.

Spread about 1-2 teaspoons of filling on one wafer, then press another wafer on top with gentle, slightly rotating pressure to spread the filling evenly.