Preheat the oven to 180 C / 350 F
Butter a 6 inch round pan and dust with flour. Line the bottom with parchment (optional)
In a blender / food processor , put in the pistachios (without shell) , the flour and baking powder
Process / grind to a fine powder . Keep aside.
In a mixing bowl, beat the butter and sugar until light , creamy and fluffy
Add the egg and vanilla .
Beat well until well incorporated.
Add the flour mixture . Beat at low speed until just combined.
Add the milk and beat until smooth.
Pour the thick batter into prepared pan and spread out evenly.
Bake at 180 C/ 350 F for 40 to 45 minutes until the edges come away from the pan, the top springs back on touch and a skewer inserted in the centre comes out clean.
Cool in pan for 5 minutes .
Gently unmold and cool right side up on a wire rack.