Pumpkin Cheesecake Brownie Bars

Make the brownies: Butter a 9-x-13" baking pan and line with parchment paper, leaving a 2" overhang on all sides. Butter the parchment. Set a rack in the middle of the oven and preheat to 350 degrees F. In a bowl set over a pan of barely simmering water, melt chocolate and butter, stirring until smooth. Set aside to cool. In a medium bowl, whisk together sugar, eggs, and vanilla. In a second medium bowl, whisk together flour, baking powder, and salt. Add sugar mixture to melted chocolate and whisk to combine. Add flour mixture and chocolate chips and gently fold until just combined and streak free. Pour brownie batter into prepared baking pan and smooth top. Bake until just set, 12 to 15 minutes. Let cool on a wire rack. Reduce oven temperature to 325 degrees F. Make the cheesecake: In the bowl of a stand mixer fitted with the paddle attachment (or a large bowl with a hand mixer), beat cream cheese on medium speed, scraping down bowl as necessary, until completely smooth, 3 to 4 minutes. Add pumpkin pie spice and salt and beat, scraping down bowl as necessary, until smooth and fluffy, 3 to 4 minutes. Add vanilla and beat 30 seconds. Add eggs one at a time, beating 1 minute after each addition and scraping down bowl as necessary. Add pumpkin, heavy cream, and sour cream and beat on low until completely smooth and streak-free, about 1 minute. Pour cheesecake batter over cooled brownies and smooth top. Bake until set around the edges and just barely jiggling in the center, 45 minutes. Let cheesecake cool to room temperature on a wire rack. Once completely cool, loosely cover cheesecake with plastic wrap and chill in the refrigerator at least 3 hours, but preferably overnight.