Preheat oven to 400 degrees Fahrenheit. Grease a baking sheet with butter and dust with flour, shaking off excess. In a large bowl, whisk together 2 cups flour, granulated sugar, baking powder, baking soda and salt. Add butter; rub in with fingertips until mixture resembles coarse meal. Mix in chocolate chips and cinnamon-sugar roasted pumpkin seeds. In a medium bowl, stir together 3/4 cup buttermilk, egg yolk and vanilla; whisk to combine. Add buttermilk mixture to dry ingredients; stir until combined — the dough will be sticky. Using floured hands, turn out dough onto a floured work surface, form into a 7- to 8-inch round, and cut into eight wedges. Use a small spatula or the side of your knife to transfer wedges to prepared baking sheet, spacing 1 inch apart. It's OK if wedges aren't perfectly shaped. Brush scones lightly with buttermilk and sprinkle with Demerara sugar. Bake until scones are golden and a tester inserted into the center comes out clean, which takes about 20 minutes. Serve warm or at room temperature.