Step 1Heat the oven to 350 degrees. Line an 18-by-12 1/2-inch jellyroll pan or large baking sheet with parchment paper.
Step 2Melt the butter in a large saucepan over medium heat. Add the cinnamon. Remove from the heat, add the chocolate and stir until it is completely melted. Stir in the sugar, vanilla and egg, and mix until smooth. Stir in the flour and beat well. Pour the batter into the pan and spread as thin and evenly as possible (it may not cover the entire surface). The mixture may have a slight elasticity, but it will even out and adhere to the parchment paper as you continue with the spreading. Sprinkle the pumpkin seeds evenly over the top.
Step 3Bake until the brittle is just set, 20 minutes. Remove from the oven and cool on a rack for 20 minutes. Break or cut into about 2 dozen rough pieces, as you would nut brittle. Store in an airtight tin if not serving immediately.