Preheat the oven to 180°C (160°C for fan-forced oven). Lightly grease and line 2 x 18cm round cake pans.
Remove the enclosed frosting sachet from the cake mix and reserve for another use. Make up the cake mix as directed on the box using the butter, milk and eggs. Divide the mixture equally between the 2 cake pans and level the tops. Bake for 20-25 minutes or until a round bladed knife inserted fully into the centre of the cakes comes out clean. Cool a little before turning out onto a rack to cool completely. Remove the baking paper. Halve each cake horizontally to create 4 cake layers of even thickness.
Sandwich the cake layers together using around a third of the frosting and all of the strawberry jam. Spread some of the remaining frosting in a thin layer over the top and down the sides of the cake to give a thin crumb coating. Chill the cake for 20 minutes.
Spread the rest of the frosting in a thicker layer over the top and sides of the chilled cake. Chill for a further 20 minutes.
To make the glaze, place the strawberry purée, sugar, lemon juice and colouring in a medium saucepan over a low heat and stir until the sugar has dissolved. Turn the heat up to medium and bring to a simmer. Cook for 8-10 minutes until thickened, stirring frequently to prevent catching. Remove from the heat and pour into a heatproof jug or bowl. You should have about 150g strawberry glaze. Leave to cool for 30 minutes, stirring occasionally, until quite thick and jammy in consistency.
Use a spoon to drizzle some of the glaze down the sides of the cake to create the dripping blood effect. Spoon the rest of the glaze on the top of the cake and spread gently with the back of the spoon or a spatula to cover completely. Serve immediately or keep in the fridge until ready to serve.