1. FOR THE TOPPING: Whisk sugars, cinnamon, salt, and butter in medium bowl to combine. Add flour and stir with rubber spatula or wooden spoon until mixture resembles thick, cohesive dough; set aside to cool to room temperature, 10 to 15 minutes.

2. FOR THE MUFFINS: Adjust oven rack to upper-middle position and heat oven to 325°F. Line standard-sized muffin pan (cups have 1/2 cup capacity) with baking-cup liners.

3. In bowl of standing mixer fitted with paddle attachment, mix flour, sugar, baking soda, and salt on low speed to combine. With mixer running at low speed, add butter one piece at a time; continue beating until mixture resembles moist crumbs, with no visible butter chunks remaining, 1 to 2 minutes. Add egg, yolk, vanilla, and buttermilk; beat on medium-high speed until light and fluffy, about 1 minute, scraping once if necessary.

4. Using 1/4-cup measure or ice cream scoop, divide batter evenly among muffin cups; using small rubber spatula, spread batter into even layer. Following photos below, break apart crumb topping into large pea-sized pieces and spread in even layer over batter (about 1/4 cup of crumbs per muffin), beginning with edges and then working toward center. Bake until crumbs are golden and wooden skewer inserted into center of muffin comes out clean, about 20 minutes. Cool muffins in pan for 5 minutes, then transfer to wire rack at least 20 minutes. Dust with confectioners sugar just before serving.

Makes 12 muffins