Preheat oven to 350 °F and grease a 9-inch springform pan. Dust pan with sugar, tapping out excess, and line bottom with parchment paper.
Pulse nuts with ½ cup flour until fine. Sift remaining ½ cup flour with salt and stir in nut mixture. Set aside.
Whip eggs, sugar and vanilla in a mixer or with electric beaters, until they hold a ribbon when beaters are lifted, about 6 minutes.
In two additions, fold in walnut mixture and pour into prepared pan.
Bake for 35 to 40 minutes, until a tester inserted in the centre of the cake comes out clean (do not open oven for first 25 minutes).
Cool cake for 15 minutes, then remove from pan and cool completely.
For custard icing, whisk milk and custard powder in a small saucepan and bring up to a simmer, whisking constantly, until thick, about 4 minutes.
Whisk in honey and remove from heat.
Strain and place bowl with custard into another bowl with ice water to halt the cooking process.
Stir in brandy and vanilla and cool to room temperature, stirring occasionally.
While custard is cooling, pulse walnut pieces and icing sugar in a food processor and set aside.
Place custard in a mixer fitted with the paddle attachment and beat to smooth. Add butter, a little at a time until all has been added, scraping down the sides of the bowl often.
Once butter has been incorporated, reduce speed and add nut mixture until blended.
To assemble the cake, cut the walnut torte in half and spread apricot jam over. Then spread about ¾ cup of walnut icing on bottom half.
Place top half over and ice top and sides, covering completely. Pipe desired garnish and gently press chopped walnuts into side of cake.
Garnish top of cake with walnut halves and chill.