Preheat your oven to 375 degrees F. Grease a 10" bundt pan or tube cake pan with butter, dust with a little flour, then tap out the excess. Set aside.

In a medium bowl, sift the flour, sea salt and baking powder together.

Using a stand mixer with paddle attachment or a large bowl and handheld mixer, cream the butter and the sugar together until light and fluffy, about 2 minutes. Add the vanilla extract and beat on low until combined, then add the eggs one at a time, beating on low to combine between each addition.

Add the dry ingredients and the milk to the butter mixture and combine on low speed. If the dough does not have a firm dropping consistency (slowly falls off a spoon) add more milk 1 tbsp at a time until this consistency is achieved. Add approximately 2/3 of the dough to the pan and spread evenly.

Add the sifted cocoa powder, 1 tbsp of sugar and 5 tbsp of milk to the remaining 1/3 of the dough and combine on low speed. Add more milk 1 tbsp at a time, if needed, to achieve a firm dropping consistency.

Spread the chocolate dough evenly over the vanilla dough, then scoop, twist and turn a fork through the bottom of the dough to the top around the entire cake pan twice to swirl the doughs together.

Bake for 40-45 minutes or until a toothpick poked into the cake comes out clean. Let the cake cool for a few minutes in the pan, then invert onto a cooling rack.

When the cake has cooled, make the glaze by whisking the icing sugar, milk and vanilla extract together in a small bowl. Place a sheet of parchment paper under the cooling rack that the cake is on to catch the drips. Drizzle the glaze over the cake using a whisk.