Grease a deep 23 x 33 cm (9 x 13 in) tin and line it with baking parchment, making sure that you leave some overhanging on either side.
To make the base, sift together the flour, baking powder and salt in a large bowl.
Add the cold butter and rub it in with your hands until it resembles fine breadcrumbs and no lumps of butter remain.
Stir in the sugar, then the vanilla extract and milk. The dough should be starting to clump together.
Tip the mixture into the prepared tin and spread it level. Pack it down as firmly as you can. I use the base of a glass to help. Prick all over with a fork
Pop the tin in the freezer to chill for 20 minutes while you preheat the oven to 180°C/160°C fan/350°F/gas mark 4.
Bake the base for 15-20 minutes until it is firm and pale golden.
While the base is baking prepare the topping. Place the butter, light brown sugar, rum and vanilla extract in a pan and stir until melted. Bring up to the boil and simmer for 1 minute.
Remove from the heat and stir in the chopped and ground nuts and a pinch of salt.
Spread the base evenly with the apricot jam then pour over the nut mixture and spread it level. Bake for about 30 minutes until golden brown.
Leave the nussecken to cool completely in the tin. I leave them overnight.
Once cool, use the overhanging baking parchment to lift the slab out of the tin. Use a very sharp knife to slice the slab into 12 squares. Cut each square in half to make 24 triangles.
Melt the chocolate, either in short bursts in the microwave, or over a pan of gently simmering water.
Dip two corners of each triangle into the melted chocolate. Shake off the excess then place them on a sheet of baking parchment until set. Store in an airtight container at room temperature.