Pour the chocolate on the acetate sheet and spread it into a very thin layer using an offset spatula. Make sure it is thin. Thick chocolate garnishes aren’t easy to cut through, and they don’t look very good.
Lift the acetate and move it elsewhere on the marble so that the acetate won’t stick to the marble from the excess at the edges.
When the chocolate is almost set, use a ruler to cut rectangles 5cm / 2in by 7.5cm / 3in using the back of a paring knife (using the blade side will cut through the acetate, making it difficult to handle the chocolate rectangles). Transfer the acetate to a flat sheet pan lined with parchment paper, chocolate side facing down. Place another sheet pan or a few nonstick rubber mats on top of the acetate to weigh down the chocolate so that the rectangles don’t bow too much once they have set. Leave them as they are until needed (the acetate will protect them from getting scuffed). When ready to plate, simply peel the acetate away from the chocolate.
Reserve in a cool place at a temperature that does not exceed 21°C / 70°F.