Preheat the oven to 350° F. Spray a 9-inch by 13-inch baking dish with nonstick spray, and then flour the pan as well.
Place the granulated sugar and lavender buds in a mini chopper or food processor. Grind them together until the lavender is thoroughly pulverized and finely dispersed in the sugar.
Whisk together the cake flour, baking powder, baking soda and salt in a medium bowl.
Put the lavender sugar and 1 cup of the butter in the bowl of a stand mixer. Beat them together on high for about 3 minutes until the mixture is fluffy. Add in the eggs one at a time, beating after each one. Beat in the vanilla extract.
Mix in the dry ingredients, alternating with the sour cream, and scraping the sides a few times until the mixture is smooth. Mix in the blueberries. Spread this batter in the prepared baking dish.
Make the streusel
Toast the chopped walnuts in a small skillet over medium heat - just for a few minutes, stirring constantly until they are fragrant.
Mix the toasted walnuts together with the all-purpose flour, brown sugar and pinch of salt in a small bowl. Melt the remaining ¼ cup of butter in a small dish in the microwave, and then stir it into the dry mixture until everything is moistened and combined.
Bake the cake
Sprinkle this streusel evenly over the top of the cake batter. Bake the coffee cake for about 60 minutes, until a tester inserted in the middle comes out clean. Remove the pan to a cooling rack and allow it to cool for at least an hour.
Slice the coffee cake into 15 pieces and serve. Leftover cake can be kept tightly covered at room temperature for up to three days.
Makes about 15 servings.
Recipe adapted from recipe in The Baker's Manual by Amendula/Rees
Look for lavender in the bulk spice section of your grocery or health food store. If it's not available locally find it online, but be sure you're getting culinary lavender. (Not a variety meant for soap and candles!)
If you have lavender already, smell it and make sure it's fresh - the scent should be very strong. If yours has faded, toss it and buy some new.
Grinding the lavender with the sugar allows the spice to be dispersed finely in the cake without chewy lumps that you would get if the lavender buds went in whole.