Begin by making the pink sheet cake. Preheat the oven to 180°C/Gas Mark 4. Line a Swiss roll tin with baking paper.
Cream together the butter and sugar until light and fluffy. Beat in the vanilla extract, followed by the eggs one at a time, beating well after each addition. Sift and fold in the flour. Add a little pink gel paste colouring to turn the cake mixture pink.
Spread out the mixture evenly in the Swiss roll tin, then bake in the oven for 15 minutes until fully baked. Allow to cool for 5 minutes in the tray, before transferring to a wire rack to cool completely. Once cooled, wrap well in clingfilm and chill in the fridge for 30 minutes.
Turn the oven down to 160°C/Gas Mark 3, then grease the base of a deep 20cm (8in) cake tin.
Prepare the batter for the main cake by creaming together the butter and sugar until light and fluffy. Add the eggs one by one, beating well after each addition. Fold through the flour and the milk until well combined.
Remove the pink sheet cake from the fridge and unwrap. Using a small heart- shaped cookie cutter, cut out about 30 pink hearts. Take a large spoonful of the main cake mixture and colour it pale pink, then set to one side.
Cover the base of the cake tin with a 2cm (3⁄4in) deep layer of plain cake mixture. Using a palette knife, create a circular trough in the batter, running about 2.5cm (1in) away from the edge of the tin. This forms a trough that the point of the heart will sit into.
Start to add the pink hearts, one at a time, spreading a little pink cake batter over one side to act as glue. Keep adding the hearts, forming a solid ring around the inside of the cake.
Using the remaining plain cake mixture, carefully fill between the sides of the tin and the hearts and in the centre, enclosing the hearts within the cake. Level the top gently with the back of a spoon, then bake in the oven for 1 hour 25 minutes, or until a skewer inserted into the centre comes out clean.
Allow to cool in the tin for 10 minutes before transferring to a wire rack to cool completely.
To prepare the buttercream, beat together the butter and icing sugar until well combined, pale and fluffy. Allow the melted white chocolate to cool a little before beating into the butter and sugar.
Once the cake is fully cooled, cover the top and sides with white chocolate buttercream using a palette knife. Sprinkle the top with pink heart sprinkles and some pink glimmer sugar.