Preheat the oven to 170 degrees C. Grease a 23cm springform cake tin.
Toast the hazelnuts in a dry frying pan over medium heat until they start to give off a nutty aroma, turning them constantly so that they don't burn. Sprinkle 40g of the caster sugar over them and continue to toast them stirring constantly until the sugar caramelises. Remove from the heat and set aside.
Beat 300g sugar, vanilla and butter until light and creamy. Add salt and lemon zest. Add eggs one at a time, beating well after each addition.
Mix flour, semolina and baking powder; stir into the butter mixture.
Pour half of the cake mixture into the springform cake tin. Spread the apricot halves over it evenly and sprinkle with the hazelnut brittle. Pour the rest of the mixture over and smooth out with a spatula.
Bake in the preheated oven for 70 minutes until a skewer comes out clean. If the cake is getting too dark towards the end of the baking time lay a piece of aluminium foil over it.
Leave to cool completely in the tin. Run a knife around the edges and remove the rim from the springform tin to serve.