For the Crumb Topping:

1. In a small bowl, mix together all the ingredients for the crumb topping.  If the mixture looks a little too saturated, add a little more flour, 1 tablespoon at a time, until large crumbs form.  Set aside.

For the Blueberry Filling:

1. In a mortar and pestle, crush the lavender buds.  In a small dish, combine the crushed buds with the sugar and lemon zest.
2. In a large bowl, toss the blueberries with flour and the lavender sugar until completely coated.  Set aside.

For the Cake:

1. Preheat the oven to 350 degrees F.   Grease and line a 9-inch cake pan with parchment paper.  Set aside.
2. In the bowl of a stand mixer, cream butter and sugar until smooth.  Add the eggs and mix again until whipped.  Lastly, add the vanilla and yogurt.  Mix again until combined.
3. In a separate bowl, whisk the flour, salt, and baking powder.
4. Add the dry ingredients to the butter mixture.  Mix on low speed until incorporated.  The batter will be thick but smooth and creamy.
5. Transfer the cake batter to the prepared cake pan.  Spread the batter evenly.  Top with the blueberry mixture evenly, and finally sprinkle the crumb topping over the entire layer of blueberries.
6. Bake for about 45-50 minutes depending on the range of your oven, or until the cake tester comes out clean.
7. Let the cake cool completely on a wire rack before transferring to a cake plate.  Dust with confectioner's sugar before serving.  Enjoy!