Preheat the oven to 180° (350°F). Mix sieved flour and cocoa powder, with a pinch of salt and butter, in an electric mixer, add the egg and keep mixing. Cover the dough with plastic wrap and cool it in the refrigerator for half an hour.
Roll the dough with a rolling pin on a floured surface until 2 mm thick, roll it on the rolling pin and transfer to the heart mould. Blind-bake for 25 minutes.
Chop the chocolate, pour the double cream in a pan with a pinch of salt and bring to simmer, then pour in the chocolate and keep stirring until well combined. Pour the chocolate ganache on the tart shell, level the surface with the back of a spoon and chill in the refrigerator for 2 hours. Decorate with Ferrero Rocher chocolates and petals.