Preheat the oven to 350°F / 175°C. Line a muffin tin with 6 parchment liners.
Add the sugar, flour, baking powder, salt and cubed butter to a medium sized mixing bowl. Use a hand mixer or stand mixer fitted with the paddle attachment to beat the mixture together until the butter has completely combined with the dry ingredients and no large lumps remain.
Add the oil, egg white, vanilla, sour cream, and milk to the flour mixture. Whisk the ingredients together just until a cohesive batter has formed.
Chai Swirl
In a small bowl, mix together the brown sugar and chai spice.
Place dollops of the chai mixture on top of the cupcake batter, then gently swirl it into the batter so that there are streaks of chai throughout the batter.
Distribute the batter evenly between the liners. Bake for 16-18 min., or until a toothpick inserted in the center of the cupcakes comes out clean.
Chai Dulce de Leche Buttercream
In a medium sized mixing bowl, beat the butter with the salt for 1 minute to remove any lumps.
Slowly beat in the powdered sugar until all of it has been used.
Add in the heavy whipping cream, dulce de leche, and chai spice. Beat the mixture for 1-2 minutes, until light and fluffy.
Once the cupcakes have cooled to room temperature, use a spoon or a piping tip to carve a hole out in the center of each cupcake. Fill each cupcake with ~1 tbsp. of dulce de leche.
Transfer the buttercream to a piping bag fitted with your piping tip of choice (I used Wilton 1M.)
Generously frost each cupcake, then garnish with more dulce de leche and half of a cinnamon stick if desired. Serve and enjoy!