Preheat the oven to 170°C (340°F) and line two baking trays with baking paper. Using a food processor with the beater attachment, mix the butter and sugar until the mixture is pale and fluffy. Add the egg and mix well to combine, then add the vanilla extract.
Into a clean bowl, sift the flour, cocoa, baking powder and salt. Gradually add this flour mixture to the butter mixture, mixing with the food processor until just combined.
Turn the dough out onto a floured surface and knead briefly until the dough is smooth. Wrap half of the dough in clingfilm and set aside. Roll the other half of the dough out onto a floured surface until it is 3–4 mm (1⁄5 inch) thick. Then, cut out heart shapes using a heart cutter in one or two different sizes. Transfer the shapes to one of the baking trays. Repeat with the remaining dough and chill in the freezer for 10 minutes (this helps the cookies to keep their shape). Bake in the oven for 8–12 minutes (depending on the size of your shapes), until firm. Allow the cookies to cool on a wire rack.
To make the decoration, melt the white chocolate in a glass bowl over a saucepan of simmering water, ensuring that the water doesn't touch the bottom of the bowl. Stir until the chocolate is almost melted, then remove the pan from the heat. Remove the bowl from the saucepan, and continue to stir until the chocolate is completely melted. Then, allow it to cool slightly. Place the sheets of transfer paper design-side up on two baking trays. Pour the melted chocolate onto the transfer paper and spread evenly over the sheet with a palate knife until it is 1–2 mm (1⁄8 inch) thick. Chill the chocolate in the fridge for 10 minutes, or until firm (but not completely set).
Press the heart cutters through the chocolate to the transfer paper. Return the baking trays to the fridge to allow the chocolate to fully set. Turn the chocolate over so that the design side is facing up, then carefully peel away the transfer paper to reveal the floral design, and push out the heart shapes. Set aside any leftover white chocolate.
To attach the chocolate paper to the cookies, melt the white chocolate that was left over after pushing out the heart shapes. Place a small amount of melted chocolate on each cookie to act as a glue, and place the chocolate hearts on top. Set aside to fully set. Store in a tin or an airtight container for up to two weeks.
This recipe is excerpted with permission from <a href="http://www.amazon.com/gp/product/1742706347/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1742706347&linkCode=as2&tag=craftfocom-20">"trEATs: Delicious Food Gifts to Make at Home"</a> by April Carter and published by Hardie Grant Books.