Preheat your oven to 160°C (fan) / 180°C (conventional) / 355°F. Grease and line two 20cm (8-inch) loose-based round cake tins with non-stick baking paper and set them aside.
In a large mixing bowl using a handheld electric mixer, or in a stand mixer fitted with the paddle attachment, beat the softened butter and golden caster sugar together for 3 to 5 minutes, or until the mixture becomes pale, smooth, and mousse-like in texture. Add the vanilla extract and mix again briefly to incorporate.
Add the eggs one at a time, beating well after each addition until the batter is smooth and fully emulsified. Sift in the plain flour and baking powder, then add the fine sea salt and the finely ground pistachios. Gently fold everything together just until the ingredients are combined—be careful not to overmix.
Evenly divide the batter between the two prepared cake tins. Smooth the tops with a spatula or the back of a spoon so the layers bake evenly.
Bake in the preheated oven for approximately 35 minutes, or until the cakes are well-risen and a skewer inserted into the centre comes out clean. Remove from the oven and allow the cakes to cool in their tins for about 15 minutes before transferring them to a wire rack to cool completely. (Note: the outer green colour may fade slightly during baking, but the inside will remain beautifully vibrant.)
While the cakes cool, prepare the pistachio buttercream. In a large bowl using a handheld mixer, or a stand mixer with the paddle attachment, beat the softened butter and sifted icing sugar together until the mixture is pale, fluffy, and has increased in volume. Add the smooth pistachio cream, sea salt, and lemon juice, then beat again to fully combine. Taste and adjust the lemon juice if needed for balance.
Once the cakes are completely cooled, place one layer on a flat serving plate. Transfer the pistachio buttercream into a piping bag and pipe half of it over the first cake in an even layer. Carefully spread the raspberry jam over the buttercream using a palette knife—keep the jam slightly away from the edges to prevent it from spilling out, though a little visible jam is desirable.
Place the second cake layer on top, then pipe and spread the remaining buttercream over the surface. Finish by decorating with fresh raspberries, a scattering of chopped pistachios, and sprigs of fresh mint. Slice and enjoy!