If you have never used porcini powder before you will be shocked at how much flavour it imparts on all manner of dishes. I found mine in Waitrose but it is not hard to find.

Ingredients
400g fettuccine 
25g butter 
3 garlic cloves, finely chopped 
250g mushrooms, coarsely chopped (these can be whatever mushrooms you like, I like to use a mix) 
150g mascarpone 
100ml cream 
2 tbsp porcini powder (optional) 
Salt 
Pepper 
200ml pasta water 

Method
Put your heavily salted water on for your pasta. 

Meanwhile melt the butter in a large frying pan or wok and add your mushrooms.  You need to cook these on a fairly high heat as mushrooms lose a lot of water during cooking process and you want them to seal and colour before the water comes out or they will stew.  Cook the mushrooms until they lose their water and the water evaporates, this will take between 5 and 10 minutes depending on the mushrooms you have picked.  

Add the garlic and cook for 1 minute, add the porcini powder if using. 

Put your pasta in to cook according to the packet instructions - fettuccine usually takes around 5 mins only.  Before draining take out a cup of water and reserve. 

Meanwhile finish off your sauce.  Add the double cream and stir then add the mascarpone and freshly ground black pepper and stir until the mascarpone has melted.  Add roughly 60ml of water and stir, you want your sauce to be thickish, coating consistency.  If it is too thick add a little more water but be aware you don’t want it to be thin too thin or watery.  Taste the sauce now and check for seasoning, remember the pasta water is heavily salted so it may be no more is needed. 

When the pasta is done stir the pasta and sauce together, put into bowls and top with parmesan and some added freshly ground black pepper.