Preheat the oven to 170 degrees Celsius/340 Fahrenheit top and bottom heat.
Place the flour, cornstarch, coarsely ground pistachios, granulated sugar, salt, baking soda, baking powder and lemon zest in a bowl and whisk to combine.
Pour the sunflower oil, butter, milk, sour cream, almond flavouring and egg into another bowl and mix for 2 minutes with a hand mixer or electric mixer.
Add the dry ingredients little by little and mix for a further 2 minutes.
Grease a baking tin with nonstick cooking spray and/or line with greaseproof paper. Pour the mixture into the tin and place in the oven.
Bake for 45 minutes, testing with a skewer - if it comes out clean, the cake is done.
In the meantime, make the glaze. Sift the icing sugar into a bowl, add the milk and mix well.
When the cake is ready, leave to cool for 20 minutes, then remove from the tin, pour the icing over the cake and garnish with the chopped pistachios.