Preheat the oven to 350 degrees F and line a 9-inch cake pan with parchment paper. I used a large piece of parchment paper, leaving the edges sticking out for easy removal.
In a medium bowl, whisk together the flour, baking soda, baking powder, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and brown sugar on high speed until smooth and light for about 2 minutes.
Add the eggs in one at a time, beating between each addition.
Add the buttermilk and vanilla, and mix until fully combined.
Add the dry ingredients, and mix until just combined.
Pour half of the batter into the prepared cake pan, then sprinkle the chai cinnamon swirl around evenly. Pour the rest of the batter on top and gently spread out evenly with a rubber spatula.
Bake the cake for 25 minutes. Then, remove and quickly add the streusel topping, gently pressing it into the top of the cake.
Bake the cake for another 40 minutes or until a toothpick inserted comes out clean.
Allow the cake to cool completely in the pan before removing and glazing.