Pre-heat the oven. Rub some oil around the inside of a metal Bundt tin or silicone mould. Measure the flour and bicarbonate of soda into a bowl, stir to combine and sieve into another bowl. Put the pistachios into a kitchen blender and pulse until they are the size of ground almonds.
Chop the butter into cubes and put them into a mixing bowl. Add the sugar and beat together until smooth. Finely grate the lime into the bowl. Break the eggs into the bowl, one at a time, ensuring each is well incorporated before adding the next.
Tip the flour into the bowl and stir to combine. Add the milk and stir again. Transfer the ground pistachios to the mixing bowl, add lime juice and mix well.
Tip the mixture into the Bundt tin or mould and smooth the top. Bake for 40-45 minutes or until the sponge springs back when touched. Leave to cool in the tin for 20 minutes then turn out onto a wire rack to finish cooling.
Meanwhile, make the pistachio and lime icing. Chop the pistachios.
Squeeze the lime juice. Measure the icing sugar into a bowl, add some of the lime juice and stir to combine.
Continue adding lime juice and stirring until the icing is just runny enough to pour. Pour the icing over the top of the cold cake allowing it to run down the sides. Scatter the chopped pistachios over the top of the cake. Allow the icing to set before serving.