Preheat the oven to 180°C (160°C fan forced). Line 2 x 12-hole muffin pans with paper cupcake cases.
Open the cake box, remove enclosed frosting sachet and reserve for another use. In a mixing bowl, stir the cake mix, eggs, butter and milk together, then whisk (by hand or using an electric mixer) for 2-3 minutes until smooth and creamy. Divide the mixture evenly among the cupcake cases. Bake both pans together in the oven for 18-22 minutes. Remove from the oven and transfer the cupcakes to a wire rack to cool completely.
Using a melon baller or teaspoon, scoop out the centre of each cupcake, reserving each scooped out piece. Place a mini chocolate egg and a few sugar sprinkles inside each hollow (reserving some for decoration). Slice some of the cake off the bottoms of the reserved pieces, then place on top of the filling, pressing down gently to give a level surface to each cupcake.
Divide the frosting equally among 4 bowls and beat in enough of each food colouring gel to give each frosting a different pastel colour. Fit a large plain or fluted piping nozzle to a disposable piping bag, and spoon in 1 colour of frosting. Pipe swirls or small blobs on top of 6 cupcakes. Repeat with the rest of the coloured frosting, removing the nozzle from the bag each time, washing it and placing in a new disposable bag.
Sprinkle each frosted cupcake with the reserved sugar sprinkles. Store any leftover cupcakes in an airtight container in the fridge for 2-3 days.