Preheat oven to 350°F. Grease and flour a 10–15 cup bundt pan.
In a medium bowl, whisk together cake flour, baking powder, and salt.
In the bowl of a stand mixer fitted with a paddle attachment, beat together butter, sugar, and sour cream until well combined.
Stir in vanilla extract until combined.
Add eggs one at a time, mixing well after each addition.
On low speed, stir in the flour mixture until just incorporated.
Stir in milk until combined.
In a separate large bowl, stir together 2½ cups of the batter, cocoa powder, milk, and red food colouring until well combined.
Using a ¼-cup measuring cup, drop two scoops of plain batter into the prepared bundt pan.
Drop one scoop of red velvet swirl batter on top of the plain batter.
Continue alternating between plain and red velvet swirl batters until both are used up.
Bake for 50–60 minutes, or until a wooden toothpick inserted into the centre comes out clean.
Cool the cake in the pan on a wire rack for 15 minutes.
Invert the cake onto the wire rack and cool completely.
Dust with powdered sugar before serving, if desired.