Preheat the oven to 180°C (160C for fan-forced ovens). Lightly grease 2 x 20cm round sandwich pans and line the bases with baking parchment.
Mix the cake mix, butter, milk, and eggs in a large bowl with electric mixer on low speed for 30 seconds, then on medium speed for 2 minutes (do not eat batter as it contains raw eggs). Fold in the lemon zest. Divide between the prepared pans and gently level the surface.
Place in the oven and bake for 25-30 minutes or until risen and golden and a skewer inserted into the centre of each cake comes out clean. Cool in the pans for 5 minutes then turn out onto a wire rack and leave to cool completely.
To make the blueberry topping, mix arrowroot and 1 tablespoon water together in a small bowl; set aside. Heat the sugar and 50ml water in a medium saucepan until the sugar dissolves. Add the blueberries and simmer for 1 minute. Stir in the arrowroot mixture and simmer, stirring for 1 minute until thickened. Leave to cool completely.
Mix the sachet of frosting for the cake mix with the tub of frosting in a medium bowl until smooth. Using a palette knife, spread some of the frosting over the top of one cold cake and spoon over 4 tablespoons of the blueberries. Place the second cake on top, pressing down gently. Spread a thin layer of some of the remaining frosting around the sides of the cake to seal in the crumbs. Chill for 30 minutes.
Spread the rest of the frosting around the sides and top of the cake. Spoon the remaining blueberries on top – don’t worry if some of the syrup drips down the side of the cake. Slice to serve.