Preheat the oven to 350F. Spray the sides of a 9-inch bundt pan with a baking spray, or butter and flour the pan very well.
In a large mixing bowl, combine the softened butter and melted white chocolate. Use a mixer or a whisk to cream the butter and chocolate together for a few minutes, until fluffy. Add the eggs and vanilla and mix again for 2 to 3 minutes. Pour in the milk and chai tea concentrate, but do not stir yet.
In a separate bowl, combine the dry ingredients: flour, baking powder and salt. Sift the dry ingredients into the batter, then mix for about a minute, just until the flour is incorporated. Spoon the batter into the prepared pan. Bake the bundt cake in the preheated oven for 45 to 50 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan.
For Chai Tea Custard:
Meanwhile, prepare the chai tea custard. In a measuring cup, combine the milk, chai tea concentrate and the cornstarch. Whisk for a minute until the cornstarch is mixed in. In a small bowl, whisk together the egg yolks and sugar for a few minutes, until creamy. Add the chai tea and milk mixture and whisk again.
Pour the resulting mixture into a saucepan and heat over low-medium heat, stirring constantly with a rubber spatula, for 4 to 5 minutes. The custard needs to thicken and hold its shape on the back of a spoon. Remove the custard from heat and whisk in the butter. Once the butter is completely melted, cover the pan with a lid and allow the custard to cool.
For removing the cake out of the pan, invert a cake stand on top of the pan, then flip over and remove the pan. Drizzle the custard over the cake and serve with extra custard on the side. If not serving right away, keep the cake stored at room temperature without the custard and well covered.