Preheat the oven to 180°C (160°C Fan, Gas Mark 4). Line a large baking tray with baking parchment. In a bowl, beat together the butter and caster sugar until soft and creamy. 

Divide the creamed mixture between 2 bowls. Add 100g (3 ½ oz) flour to one bowl and sieve the cocoa on top. Mix the ingredients together to form a firm, chocolatey dough. 

Add the remaining flour to the remaining creamed butter and sugar and mix together to form a firm dough.

Working on one dough at a time, turn on to a lightly floured surface and knead gently until smooth. Roll out to a thickness of ½ cm ( ¼ inch) - the mixture will be quite short. Using a 3.5cm (1 1/3 inch) wide heart-shaped cutter, stamp out 32 heart shapes, re-rolling as necessary. 

Repeat with the remaining batch of dough to make a further 24 heart shapes.

Arrange all the heart shapes on the baking trays, spaced slightly apart. Prick the tops with a fork and chill for 30 minutes.

Bake for 12-14 minutes until lightly golden and firm to touch. Cool for 5 minutes then transfer to a wire rack to cool completely. 

Spread the Chocolate Buttercream over 6 plain shortbread hearts and spread the Vanilla Buttercream over 6 chocolate shortbread hearts, right to the edges, and sandwich together with the remaining shortbread hearts. Arrange on a board lined with baking parchment.

Use any leftover buttercream to stick on the White and Milk Chocolate Hearts. Leave for a few minutes in a cool place to set before serving.