Sea Level baking time per recipe: 35-40 minutes.
Baking time at 9,000 feet: 45 minutes.

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.

FOR THE CAKE: Adjust oven rack to upper-middle position and heat oven to 325 degrees. Cut 16-inch length parchment paper or aluminum foil and fold lengthwise to 7-inch width. Spray 8-inch square baking dish with nonstick cooking spray and fit parchment ingo dish, pushing it into corners and up sides; allow excess to overhang edges of dish.

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 in tablespoon increments, continue beating until mixture resembles moist crumbs with no fisible butterchunks 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.

Transfer batter to baking pan; using rubber spatula, spread batter into even layer. Break apart crumb topping into large pea-sized pieces and spread in even layer over batter, beginning with edges and then working toward center. Bake until crumbs are folden and wooden skewer inserted into center of cake comes out clean. Cool on wire rack at least 30 minutes. Remove cake from pan by lifting parchment overhang. Dust with confectioners’ sugar just before serving.