1.Preheat oven to 190°C/375°F. Grease and line base and sides of a 20cm (10in) square cake pan.
2.Roast hazelnuts on an oven tray for 10 minutes or until golden. Cool; process to coarse crumbs.
3.Melt chocolate and butter in a heatproof bowl over a saucepan of simmering water (don’t allow bowl to touch water), stirring occasionally, until smooth and combined. Remove from heat; stir in buttermilk and egg yolks.
4.Meanwhile, whisk egg whites and cream of tartar with an electric mixer until frothy. With motor operating, gradually add honey; whisk until glossy firm peaks form. Working in batches, fold in chocolate mixture, hazelnuts, cocoa and salt.
5.Pour batter into cake pan; bake for 30 minutes. Cool in pan, pressing top of cake gently as it cools so it falls back evenly. Turn cake onto a board.
6.Top sliced cake with the fresh raspberries; serve.