Heat oven to 180C/160C fan/gas 4 and line two 12-hole cupcake tins with paper muffin cases. Put all the cake ingredients in a large bowl and whisk with an electric whisk for 3-4 mins or until pale and light. Divide the mixture evenly among the 24 cases and bake for 20 mins or until a skewer inserted into the centre of a cake comes out clean. Leave to cool on a wire rack.
While the cupcakes bake, make the syrup. Put the sugar, lavender and 300ml water in a medium pan over a low heat. Bring gradually to the boil, cook for 2 mins, then turn off the heat and leave to infuse. When cold, strain through a sieve into a small jug.
To make the icing, put the butter, icing sugar and 100ml of the syrup in a large bowl. Mix with a wooden spoon, then switch to an electric whisk and whisk until smooth and fluffy. Divide the mixture between three bowls. Add a tiny dot of food colouring gel to each bowl to make three different colours of buttercream – stir to mix, then transfer half of each colour to disposable piping bags.
Use a cocktail stick to poke a few little holes in the top of the cupcakes, then drizzle each with 1 tsp of the syrup. Put 1 heaped tsp of icing onto each cake (do eight of each colour) and swirl it round with the back of the spoon to completely cover the top. Next, use the matching piping bag to pipe a straight line about 2-3 cm long across the middle of each cake.
To make the wings, melt the white chocolate in a heatproof bowl over a pan of simmering water. Divide into three smaller bowls and add the same gel colours as before. Cut 8cm squares of baking parchment and, using a pen, draw a pair of butterfly wings on each square so that the design shows through on the other side or, if you prefer, trace the design below. Turn the paper over and, using the line as a guide, paint on the melted chocolate. Sprinkle crushed biscuits on the edges to create patterns, then leave to set. Once set, gently push the wings into the cupcakes at a 45-degree angle.