You can prepare it the day before or well in advance and freeze it.
Preheat oven to 180°C ( 350°F ) fan-assisted (optional)
Grease two baking sheets and draw a square with the pastry frame on parchment paper.
Place the drawn side against the baking sheet. grease and flour.
Beat the egg whites until stiff peaks form, then gradually add the sugar in three additions, continuing to beat to stabilize the meringue.
Gently fold in the egg yolks one at a time with a spatula. You can use a mixer, but run it for just a few seconds to avoid deflating the mixture.
Work from bottom to top, turning the bowl. Gradually fold in the sifted flour and cocoa powder in three additions. Mix gently.
Pipe the mixture into a piping bag or a freezer bag, then pipe lines next to each other inside the square. Create a new line around the square, expanding it outward.
Smooth with a spatula and dust with powdered sugar.
Bake for about 15 minutes. Check for doneness by inserting a toothpick inserted into the center. The ladyfinger should remain soft.
Turn out onto a towel, remove the parchment paper, and then transfer to a wire rack. Leave to cool, then cut 2 squares with the baking frame ring. Keep the scraps to make verrines. You can freeze them for later use.