Ok, that's about two sticks of butter, so I would use two ounces of trim/larfy nugs, should do the trick. It says for the frosting you should use another stick of butter, so if you wanted to, you could just make three sticks worth of weed butter and use three ounces of trim. Maybe throw in some hash/kief... up to you.
OK Guys, im starting to understand

Just one more question,
I found these 3 methods on the net.
Cannabutter #1
Ingredients: 1 lb (450 g) butter
1 oz (28 g)good ganja or hash or 2 oz (57 g) leaves
Method: Melt the butter slowly in the pan. Grind down your ganja to a fine powder and sift out any stalky bits. Gently add to the melted butter and stir in well. It will turn dark green if you're using grass.
Keep on a low heat for up to a half an hour without letting it burn, giving it plenty of stirring.
Then pour through a fine strainer squeezing all the butter into a jug. You can use the mush in a drink or simply compost it. Pour your Cannabutter into jars and seal them tightly for storage in the fridge. If you cover the hardened butter with a little water, this will help it keep a little longer.
Cannabutter #2
Melt 0.5 kg of butter or ghee in a saucepan. Add several ounces of finely sifted marijuna. Simmer and stir for a few minutes until the butter takes on a greenish color of the grass.
Pour the butter through a fine stainer. Press the mash firmly to squeeze out as much butter as possible. A little heat may be applied beneath the mash to help the butter flow better.
Do not discard the leafy material. Simmer it in (hot) milk or vodka and sweeten with honey to make a tasty and effective beverage.
For extra potency the process may be repeated by heating more marijuana in the same butter. Otherwise do as above.
To store cannabutter may be frozen or kept for a long time in the refrigerator by pouring water over it.
A simpler and more potent cannabutter can be done by using hashish or hash oil instead of marijuana. Heat and stir until all the hash or oil dissolves in the butter. As much hash or oil may be used as desired as it will dissolve very easily.
Cannabutter #3
Fill a large pot 1/3 the way with crushed and crumbled bud and leaf. Fill the pot 3/4 way with a mixture of 1 part butter and 4 more parts water. Boil for 30 minutes stirring frequently.
Remove as much debris from the bottom as possible. Allow the liquids to cool to room temperature,then refrigerate. The butter will harden on the top and can be removed. Be gentle to keif layer. Discard the water.
If a stronger product is desired the process can be repeated with fresh water and grass.
The one i like best is method 2 because no butter is involved and that means no separation and less time, im cooking the cake straight after the butter...
If by using method 2 i let it cook on low flame for a very long time so the THC gets absorbed properly, Would i still be wasting some or would it give the same results as method one(Water Method)?