Start by making the crème patissière. In a large bowl, whisk together the sugar and egg yolks until lighter in colour and creamy. Mix the corn flour with the 2 tablespoons of cold milk until combined. Add to the egg mixture and give it a good mix.
Pour the 200ml of milk into a pan and add the vanilla pod and seeds. Heat gently over a low heat until bubbles appear at the side of the pan. Take off the heat and pour the milk through a sieve into the egg mixture, whisking all the time.
Pour the mixture back into the pan and cook gently over a low heat, whisking constantly to avoid lumps. The mixture will soon begin to thicken. Once thick enough to coat the back of a spoon, take off the heat, add the butter and give it a good whisk until nice and smooth and all the butter has melted. Pour into a bowl, cover the surface with cling film to avoid a skin forming and cool in the fridge.
Make the shortbread. Preheat the oven to 200 C/fan 180 C/gas 6. Draw a 20cm square on a piece of baking parchment, turn it over and place the paper on a greased baking tray.
Put the flour, butter and sugar in a bowl and beat until the consistency is like breadcrumbs. Beat a little more until everything comes together into a ball. Press the mixture on to the baking parchment within the marked square and bake for 12–15 minutes or until pale in colour and firm to the touch. (Leave the oven at this temperature.)
Leave on the tray for 3–4 minutes, then square off the four edges and cut the shortbread into 32 even triangles. Do this by cutting out a square grid of 4 x 4 squares, then cutting lines diagonally across the square until you have enough triangle shapes. Dust with caster sugar and leave on the tray to cool completely. It is essential that you slice the shortbread while still warm. If left until cold, the shortbread would crumble. For sharper edges you could score the triangles on to the shortbread before you put it in the oven.
Make the cake. Grease and line the base of a 20cm square loose-bottomed cake tin.
Beat the butter and sugar together with a wooden spoon or electric whisk, until light and creamy. Gradually add the egg, beating well and adding a tablespoon of the flour to help prevent curdling. Fold in the remaining flour, the ground almonds and buttermilk. Gently mix until well combined.
Pour the mix into the tin and bake for 20 minutes or until a skewer inserted in the centre of the cake comes out clean. Leave in the tin for 10 minutes to cool a little before carefully turning out on to a wire rack to cool completely (it’s quite a fragile cake so handle it carefully).
To assemble, put the crème patissière into a piping bag fitted with your favourite nozzle and pipe even lines over the top of the cake, alternating with lines of the shortbread biscuits, pointing up. Dot the raspberries over the crème patissière and dust with icing sugar.