Make the topping: In a medium bowl, mix the flour with the oats, brown sugar, chai, and salt. With your hands, rub the butter into the mixture until incorporated, pressing it into small clumps. Freeze until chilled, about 5 minutes.
Meanwhile, make the batter: Line a 12-cup muffin pan with paper or foil liners. In another medium bowl, whisk the flour with the baking powder, baking soda, and salt. In a stand mixer fitted with the paddle attachment, beat the 1 stick of butter with the granulated sugar and chai at medium speed until fluffy. Beat in the eggs one at a time, then beat in the sour cream and vanilla. Scrape down the sides of the bowl, then gradually add the dry ingredients and beat until just incorporated.
Using a 3- to 4-tablespoon ice cream scoop, scoop the batter into the prepared muffin cups; top with the chilled topping. Bake until a toothpick inserted in the center of a muffin comes out clean, about 25 minutes. Transfer the muffins to a wire rack to cool. Dust with confectioners’ sugar before serving.