Start with your ingredients:
1/3 cup water
4 cups sugar
2 cups light corn syrup
4 oz. unsalted butter (room temp)
4 cups heavy cream (scalded)
1/2 tsp salt
1 Tbsp vanilla
Then place your water, sugar, and light corn syrup in a large pot. I would recommend something deep so when you pour the cream in later you have a lot of room for bubbling and expansion.
Cook until sugar reaches 305 degrees F. Washing down the sides of the pot with a brush and water to prevent crystallization. If you don't want to wash down, put a lid over the pot and check the temperature periodically, the steam will naturally keep the sides moist and free of crystals.
Once the sugar mixture reaches 305 degrees F, remove from heat and add scalded cream and room temperature butter slowly. It will bubble up and look something like this. (above photo)
Place back on the heat after adding salt and vanilla and cook until 248 degrees F. The caramel will be fairly light in color but extremely bubbly. Be careful not to burn yourself as sugar burns can be quite painful. Use oven mitts and a steady hand when handling hot sugar.
When caramel has stopped bubbling, pour it into a half sheet pan prepared with foil or industrial plastic wrap that won't melt. Let cool and cover overnight. Cut and wrap the next day.