Grease an approximately 40 x 35 cm (15.74 x 13.77 inch) baking tray (baking sheet) with butter.
Preheat the oven to 180°C (356 °F)with both upper and lower heat (Convection bake function: 160 °C (320 °F)).
To make the shortcrust pastry, combine flour and baking powder in an empty bowl and mix well. Add the remaining shortcrust pastry ingredients and knead into a smooth dough.
Transfer the dough to the prepared tray and lightly dust it with flour.
Using your hands, dusted with flour, press the pastry into the tray until the bottom is covered. Keep dusting your fingers with flour since the dough is a bit sticky.* If you prefer, you can first roll out the dough on parchment paper and then transfer the parchment paper to the prepared tray. Prick the pastry several times with a fork and bake for about 6 minutes.
Meanwhile, make the chocolate batter. For the chocolate cake batter, combine plain flour, baking soda, unsweetened cocoa powder, baking powder, and spices and sift. Cut the chocolate into small pieces.
In an empty bowl, add eggs, sugar, a pinch of salt, butter, and vanilla, and beat until creamy.
Add half of the flour mixture and stir well, followed by half of the buttermilk. Repeat the process until you have used up all the flour and buttermilk. Fold in chopped nuts, chocolate, and lemon zest.
Remove the baking tray/baking sheet with the shortcrust pastry from the oven. Spread the chocolate dough over the baked shortcrust pastry, smooth it out, and bake for about 25 minutes.
Remove the cake from the oven and let it cool down completely. Once the cake has cooled down, add roughly chopped chocolate and whipping cream to a microwave-friendly bowl and melt the chocolate. You could also melt the chocolate over a hot water bath.
Mix well and spread the chocolate glaze over the cake and sprinkle with sugar sprinkles. Let the chocolate glaze dry for at least 30 minutes and enjoy.