Preheat the oven to 350 F.
Lightly oil an 8×8-inch pan and set aside.
To make the crumble topping, add all of the ingredients to a medium-sized bowl, add the cold butter last. Mix into crumbles using your hands. Be quick and don't overdo it or the butter will melt from the warmth of your hands.
Using a stand mixer, beat the sugar, egg, and sourdough starter on medium-low speed for 1 minute. Add the soft butter. In a separate bowl, sift the dry ingredients. Add to the wet ingredients and mix on low speed until just combined. Finally, add the yogurt and mix until smooth.
Pour the batter into the pan. In an even layer, scatter the blueberries evenly across the surface. Then cover with the crumbles.
Bake at 350F for 45-50 minutes or until a toothpick comes out clean when inserted (baking time may vary by oven). Cool completely before serving. Dust with powdered sugar, if desired.
