Preheat oven to 350˚F (177˚C), then grease and flour a 9-inch (23-centimeter) Bundt pan.
In a medium bowl, whisk together the flour, salt and baking powder.
In a large bowl, beat the eggs and granulated sugar for approximately 5 minutes, until light and fluffy. Slowly add the vegetable oil and vanilla, beating to combine. Then, add the flour mixture a little at a time, beating between additions. Add the milk and beat to combine.
Pour two-thirds of the cake batter into the prepared pan. Add the cocoa powder to the remaining one-third of the batter and stir until well combined.
Spoon the cocoa batter on top of the plain batter and swirl lightly with the tip of a knife. Bake for approximately 50 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake cool completely and dust it with powdered sugar (if using) before serving.