Special equipment: a 12-cup swirled bundt cake pan, such as Nordic Ware Heritage Bundt Pan, 2 large resealable plastic bags and a long wooden skewer
Position a rack in the lower third of the oven and preheat to 350 degrees F. Spray a 12-cup swirled bundt cake pan, such as Nordic Ware Heritage Bundt Pan, with cooking spray.
Whisk together the flour and salt in a medium bowl. Whisk together the milk, vanilla, eggs and yolks in a second medium bowl.
Beat the butter in a large bowl with an electric mixer on medium speed until smooth, about 1 minute. Gradually pour in the granulated sugar, 1/4 cup at a time, and beat until light and fluffy, about 5 minutes.
Turn the mixer to low and add the flour mixture 1/4 cup at a time until fully incorporated, about 3 minutes. Add the egg mixture and beat until the batter is combined, about 3 minutes.
Spoon 4 cups of the batter into a large resealable plastic bag. Add the cocoa powder to the bowl with the remaining batter and beat on low speed until combined, about 1 minute. Spoon the chocolate batter into a second large resealable plastic bag.
Snip the corners off the bags. Working in alternating colors, pipe strips of batter into the ridges of the pan (the pan has an odd number of ridges so you will have two of the same batters next to each other). Use a long wooden skewer to spread the strips of batter in the ridges, taking care not to let batter spill over into the neighboring color. Pipe the rest of the batter into the pan in large dollops, alternating flavors as you go. Gently smooth the top of the batter with an offset spatula.
Bake until a skewer inserted into the center comes out clean and the top springs back when lightly pressed, 55 to 60 minutes. Cool the cake on a rack for 10 minutes. Invert the cake onto the rack to cool completely, about 2 hours.
For the glaze: Meanwhile, whisk together the confectioners' sugar and milk in a medium bowl to make a thick but pourable glaze, adding a little more milk or sugar to get the right consistency. Drizzle the glaze into the ridges of the cake.
When measuring flour, we spoon into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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