Preheat oven to 350°F/175°C. In a large bowl, cream together the butter and brown and white sugar till smooth.
Beat in the eggs, one at a time, and mix in the vanilla, salt, and baking soda, blending well.
Stir in the cocoa powder and fold in the flour.
Knead gently into a dough.
Take a small amount of the cookie dough, roll into a golf size ball, and press into the wells of a greased and floured muffin tray. Take care not to press the dough too thin.
Bake for 8-10 minutes. Remove from the oven and using the back of a teaspoon, press down into the centre to make wells.
Allow to cool in the pans for 5 minutes before transferring to a cooling rack. While the cookie cups are cooking, prepare the chocolate filling.
Chocolate Pudding Filling
In a small saucepan, combine the cocoa powder, sugar, salt, and cornstarch.
Turn the heat on to medium and slowly pour and blend the milk into the dry ingredients, stirring continuously.
Allow to boil for one minute only, stirring as it thickens, then remove from heat.
Add butter and vanilla essence, stirring well into mix.
Pour into the cookie cups and allow to set on the counter for 10 minutes (or in the fridge). Dust with sugar icing or chocolate powder.