8tablespoonsunsalted butter, 1 stick, melted and still warm
1 3/4cupscake flour, 7 ounces
For the Muffins:
1 1/4cupscake flour, 5 ounces
1/2cupgranulated sugar, 3 1/2 ounces
6tablespoonsunsalted butter, cut into 6 pieces, softened but still cool
1large egg yolk
Confectioners’ sugar for dusting
For the topping: In a medium bowl, stir together sugars, cinnamon and butter until combined. All flour and stir until mixture forms a thick, cohesive dough; set aside to cool 10 to 15 minutes.
To make the muffins, preheat the oven to 325 degrees F. Line muffin cups with muffin liners
In bowl of stand mixer fitted with paddle attachment, on low speed, mix flour, sugar, baking soda, and salt to combine. While the mixer is running on low speed, add butter one piece at a time; 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.
Divide batter equally among prepared muffin cups. Using both hands, break apart the crumb dough into small pea sized pieces. Roll the broken dough between your thumb and forefinger to form crumbs. Sprinkle crumbs evenly over the cake batter, breaking apart any larger chunks, using about 1/4 cup of crumbs per muffin.
Bake until crumbs are golden and toothpick inserted into center of muffin comes out clean, about 20 minutes. Cool muffins in muffin tin for 5 minutes, then transfer to wire rack and cool 5 minutes before serving. Dust with confectioners’ sugar just before serving.