Sift the flour, cocoa powder, baking powder and salt into a mixing bowl. Add the sugar, Trex, unbeaten eggs, milk and vanilla extract. Use a wooden spoon to beat the mixture together until smooth and creamy — this will only take one to two minutes.
Spoon the mixture into the paper cases. Transfer to the middle shelf of the oven and bake for 22-25 minutes, until risen and firm. Cool in the tin for a few minutes, then transfer to a wire rack to cool.
To decorate, beat the butter, icing sugar and cocoa powder together for three to four minutes until light and fluffy. Pipe or spread the topping onto the cakes, leaving a slight depression in the middle. Sprinkle chocolate flake around the edges, then place 3 chocolate eggs in the middle of each cake.