In a large pot, heat brown sugar, granulated sugar, honey, water and salt, over medium heat. Stir every five minutes or so. Using a candy thermometer, continue to heat the mixture until it reaches exactly 302 degrees Fahrenheit. This is very important because this is the temperature at which sugar hardens into a rock-like state after it cools. Pay close attention not to go above 302 degrees, otherwise you will burn the sugar. It can take some time for the mixture to reach temperature, so be patient.
Meanwhile, line a baking sheet with parchment paper. Grease the parchment paper, and set aside. Place the butter, cashews, almonds, pumpkin seeds and cranberries into a large mixing bowl and set aside.
Once the sugar mixture reaches 302 degrees, immediately remove it from heat. Using a heavy wood spoon, stir in the butter, cashews, almonds, pumpkin seeds and dried cranberries, until they're coated in the mixture.
Immediately pour the mixture over parchment paper, and spread it out into a large rectangle using the back of the wood spoon. Try to keep the surface relatively even, about a 1/2-inch in height. Place the pan in the refrigerator and allow the brittle to cool for one hour. Once it has finished cooling, remove the sheet of brittle from the parchment paper and break the brittle into pieces using a meat tenderizer or clean hammer. Arrange the pieces on a serving platter and serve. Store excess brittle in a cool dry place.