Put the chocolate and 50ml cream in a heatproof bowl set over a pan of barely simmering water. Stir occasionally until melted, then leave to cool to room temperature.
Use an electric hand whisk to beat the remaining cream and half the lemon zest into the chocolate cream, until combined. Transfer the ganache to the fridge for 1 hour, or until needed.
Put the butter and sugar in a bowl and use an electric hand whisk to beat together, until light and fluffy.
Add the flour, ground almonds, remaining lemon zest and a pinch of salt, then bring the mixture together to form a dough. Divide in half.
Grease and line 2 large baking trays with baking paper. Place half the dough on a lightly floured surface and roll out to 0.5cm thick. Use a cookie cutter to stamp out 10 x 5cm rounds and arrange on one of the trays. Repeat with the remaining half, stamping out a small heart shape from the centre of each round. Transfer to the fridge to chill for 30 minutes.
Preheat the oven to 170°C, gas mark 3. Bake the biscuits for 12-15 minutes, or until light golden. Remove from the oven and transfer to a wire rack to cool completely.
Spread the whole biscuits with the ganache. Add ½ tsp lemon curd to the centre of each, top with the heart window biscuits and transfer to the fridge to chill until you are ready to serve or gift.