SPONGE
Start by mixing together all the dry ingredients in a bowl.
Warm up the milk at low heat and melt the coffee powder until uniform and liquid.
Add the melted coffee in milk to the bowl with dry ingredients, slowing adding the remaining wet ingredients and mix well.
Pour in a 21 cm cake tin and cook for 25/35 mins at 180 degrees.
Double check the cooking by using a spaghetto to perforate the whole depth: the spaghetto should come out almost dry.
Leave the cake to cool down for an hour at least, until is firm enough to be sliced in half for the filling.
FILLING
In a saucepan, start by slowly melting the double cream and coffee powder.
Now add the sugar, the vanilla, chopped walnuts and the starch making sure that all melts properly.
Once the cream starts its solidification, add the whole egg and stir constantly until the cream becomes dense enough (see pic)
Leave to cream at room temperature until it releases any steam and spread a generous layer in on the cut cake leaving a couple of spoons for the topping.
Decorate with whole walnut halves.
... Buon appetito!