Mix the cake flour, sugar, baking powder, baking soda and salt together.
Blend in 1 tablespoon of the slightly softened butter at a time on low speed until all 6 have been incorporated with no visible signs of butter lumps . The mixture should resemble large moist crumbs.
Add the 2 egg yolks, vanilla, yogurt and buttermilk. Mix on medium-high speed until it appears light and fluffy, about 1 minute. Scrape the sides as needed.
In an 8' x 8" pan lined with 2 alternating strips of parchment paper, spread the batter evenly using a spatula.
Break up the firmed topping into crumbs by rubbing it between your forefingers into pea-sized clumps. Spread the crumbs out beginning around the edges and working towards the middle. Evenly sprinkle over the batter any remaining crumbs.
Bake in a preheated 325°F oven for 35-40 minutes or until toothpick inserted in center comes out clean. Allow to cool for 30 minutes on cooling rack, then lift it out using the parchment paper. Dust it with powdered sugar and cut for serving. Enjoy!