High Altitude Chocolate Snickerdoodles

Ingredients

• 2 tsp vanilla extract
• 2 large eggs
• 3/4 tsp baking soda
• 1 tbsp corn starch
• 3/4 cup granulated sugar
• 2 tsp cream of tartar
• 2 2/3 cups all-purpose flour, (fluffed, spooned and leveled)
• 3/4 tsp coarse Kosher salt ((if using table salt, use half the amount))
• 1 cup unsalted butter, (softened to room temperature)
• 1/4 cup cinnamon-sugar ((1/4 cup granulated sugar + 1 tsp ground cinnamon))
• 1/3 cup light or dark brown sugar, (lightly packed)
• 1 egg yolk ((save the extra white for another use))
• 1/3 cup unsweetened Dutch-processed cocoa powder