Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease and base-line 2 x 16cm round cake tins with greaseproof paper.
Mix the brownie mix, oil, water and egg together gently in a medium bowl and whisk (by hand or electric mixer) for 2-3 minutes, until well blended (mixture may be lumpy).
Divide the mixture evenly between the 2 cake tins. Leave in the tins for 5 minutes then turn out onto a wire rack and leave to cool completely.
Bake in the centre of the oven for 20-25 minutes or until a rounded knife, inserted 5cm from the edge of the tin comes out almost clean. Leave in the tins for 5 minutes then turn out onto a wire rack and leave to cool completely.
Place the icing in a small bowl and beat in enough food colouring paste or gel to give an even pink colour. Peel off the greaseproof paper lining from the brownie cakes and place one on a serving plate. Spread over half of the icing then place the second brownie cake on top. Spread the remaining icing on the top brownie cake and swirl with the tip of the palette knife.
To make the marshmallow ‘petals’, cut each marshmallow in half diagonally with small scissors. Place the sprinkles on a flat plate and dip the cut side of each marshmallow half into the sprinkles.
Place 3 chocolate chips in the middle of the cake, spaced in a triangle, then place 6 of the marshmallow ‘petals’, cut side up, around each chocolate chip to create 3 flowers. Repeat with the rest of the chocolate chips and ‘petals’ to create 6 more flowers around the edge of the cake.