Preheat the oven to 350°F / 175°C. Generously butter and coat a 12-cup capacity bundt pan with granulated sugar.
In a medium-sized bowl, whisk together the flour, baking powder, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat together the butter, oil, and sugar for 5 minutes, until light and fluffy.
Scrape down the sides of the bowl, and add in one egg at a time, beating for 20 seconds between each addition. Add in the vanilla with the last egg.
Add in half of the flour mixture, and fold until just combined.
Pour the cold brew milk through a fine mesh sieve into another cup or bowl. Pour 50g of the cold brew milk into a separate bowl and place it in the fridge.
Pour the remaining milk into the batter and mix until combined. Then, add in the remaining flour and fold until just combined.
Remove half of the batter and place it into another mixing bowl. To one bowl, gently mix in the sour cream. To the other bowl, gently mix in the pumpkin puree and pumpkin pie spice.
Spread ⅓ of the vanilla batter into the bundt pan, followed by ½ of the pumpkin batter. Top with ½ of the remaining vanilla batter, followed by the rest of the pumpkin batter, and the rest of the vanilla batter.
Use a knife to gently swirl the batters together in the bundt pan.
Bake the bundt for 40-50 minutes, until a knife inserted in the bundt comes out clean or with only a few moist crumbs.
Let the cake cool in the pan for 10-15 minutes, then flip the cake out onto a cooling rack to let it come to room temperature.