Preheat the oven to 350 degrees F. Line a 9X13-inch pan with foil for easier cleanup (I use metal; glass will bake differently and you may want to decrease oven temp to 325 degrees). Lightly grease foil and sides of pan with nonstick cooking spray. Set aside.
For the brownie batter, in a large microwave-safe bowl, combine the chocolate chips and butter and microwave in 1-minute increments, stirring in between, until melted and smooth. Don't over heat!
Stir in the sugar and whisk until smooth. Add the eggs and vanilla and mix until well-combined. Stir in the flour, cocoa and salt until no dry streaks remain and batter is smooth. Set aside. Making the brownie batter first gives it a chance to firm up a bit while you make the snickerdoodle batter.
For the snickerdoodle batter, in a large bowl, add the softened butter, granulated sugar and brown sugar. Mix with an electric mixer (stand mixer or handheld) until smooth and creamy, 1-2 minutes. Add the eggs and vanilla and mix until well-combined.
Add the flour, cinnamon, salt and baking powder and mix until evenly combined and no dry streaks remain.
Scoop heaping spoonfuls (about 2-3 tablespoons in size) of brownie batter onto the bottom of the prepared baking dish, leaving space in between. Use about half or 2/3 of the brownie batter; set the remaining batter aside.
Scoop spoonfuls of the snickerdoodle cookie batter in between the brownie batter. Use all of the snickerdoodle dough.
Spoon the rest of the brownie batter over the top in small streaks or spoonfuls.
Leave the batter as is OR lightly spread with a spatula (don't press on the dough - just lightly spread so the top is evened out a bit).
Combine the sugar and cinnamon for the topping and sprinkle it evenly on top of the bars.
Bake for 30-35 minutes until the bars are puffed (the top of the bars may be bumpy and splotchy and uneven - that's perfect!). A toothpick inserted in the center may come out with moist crumbs but shouldn't have gooey batter. Add time as needed.
Let cool completely on a wire rack. Refrigerate for an hour or so to completely chill if possible (this will make cutting the bars a lot easier). Cut into squares and serve chilled or at room temperature. Bars freeze well.
Notes
Chocolate Chips: you can use bittersweet chocolate chips in place of the semisweet but I wouldn't recommend using milk chocolate. The best chocolate chips, in my opinion, for melting and flavor are Guittard or Ghirardelli.