Lightly oil two 15½-inch-long jelly roll pans or a clean, flat work surface, preferably of marble or another cool material.
Place the butter in a large heavy-bottomed saucepan over medium heat. When melted, stir in the sugar and corn syrup and raise the heat to high. Clip a thermometer to the side of the pan and cook, stirring constantly, for about 4 minutes, or until the mixture comes to a boil. Continue to boil, stirring constantly, for about 15 minutes, or until the mixture is golden brown and registers 315°F on the thermometer.
Remove from the heat and stir in the salt and baking soda, taking care, as the soda will cause the mixture to boil up. Stir in the vanilla. Add the pumpkin seeds and cranberries, stirring to thoroughly blend.
Pour an equal amount of the hot mixture into each oiled pan (or onto the oiled surface), spreading it out to about ¼ inch thick with an offset spatula.
When the brittle has set, use a sharp chef’s knife to crack it into individual pieces.
Place one or two wire racks on parchment paper.
Place the tempered chocolate in a warmed bowl and, using tongs, dip each piece of brittle into the chocolate, lifting it up to allow the excess chocolate to drip off. Place the chocolate-dipped brittle on the wire racks to set. You may have to dip a couple of times to completely cover the candy. You may also dip only half of each piece. If so, dip on the bias for a more attractive pattern. Let stand for about 30 minutes, or until the chocolate has hardened completely.
Serve immediately or store layered in an airtight container, for up to 1 week.