Preheat the oven to 175 C. Line a 9x5 loaf tin (see notes) with non-stick baking paper, leaving a little overhang for easy removal. Set aside.
In a mixing bowl, beat the butter, sugar and lemon zest until pale and creamy, using a hand mixer on medium speed.
Add the eggs and vanilla and beat again to combine. Sift in the flour, baking powder and salt (if using) and gently stir into the batter.
Add the ground pistachios and almonds and use a spatula to stir them into the batter until combined, but do not over-mix. Finally, stir in the milk to make a thick but spreadable batter (see photos below).
Spoon into the prepared tin, smoothen the top, sprinkle on the sliced almonds and bake for 40 to 45 minutes or until a toothpick poked in the centre comes out with only a few moist crumbs.
Allow the cake to cool in the tin for 20 minutes, then lift the cake out using the paper overhang and set it on a wire rack. Gently peel away the paper from the sides and allow it to cool completely before slicing into 8 thick pieces or 16 smaller ones. In humid weather, this cake is best stored in the fridge (a week to 10 days), otherwise it will keep at room temperature in an airtight tin for 3 to 4 days. Enjoy!