For the cupcakes:
Center the oven rack and preheat the oven to 350 F (180 C).
Line two 24-cup muffin tins with paper liners.
Combine the flour, cocoa, baking powder, baking soda, and salt. Set aside.
Cream the butter and sugar until smooth.
Add the eggs one at a time, mixing well after each addition, then add the vanilla and mix.
Reduce the mixer speed and alternately add the reserved dry ingredients and buttermilk.
Mix in the melted chocolate.
Divide the batter among the muffin cups and bake for 22 to 25 minutes.
Let cool for 5 minutes in the pan before transferring to a wire rack to cool completely.
Glaze:
In a bowl, combine all the ingredients and beat with a whisk until smooth and homogeneous.
Set aside about 2 tablespoons (30 ml) of white glaze and reserve it in a small bowl, covered with plastic wrap.
Divide the remaining glaze into 4 bowls and add the food coloring, mixing well to obtain uniform colors. Cover the bowls with plastic wrap.
Sort the Smarties: you will only need the black ones for the eyes, the orange, yellow, blue, and green ones for the scales, and the red ones for the mouth.
Cut the Petit Beurre biscuits into isosceles triangles and insert the pointed part into each cupcake, pushing it halfway in. This will form the tail of the fish.
Glaze the cupcakes and the biscuit with the colored glaze, smoothing it out with a small spatula. It should be about 1/4 inch (0.5 cm) thick.
Place the Smarties corresponding to each glaze color on half of the cupcakes.
Using a pastry bag, make a point of white glaze for the eye and place a black Smarties in the center.
Place 2 red Smarties to form the mouth.
Present your cupcakes on a large platter, arranging them in the shape of a large fish.