Preheat the oven to 350 degrees F. Butter and flour a 9-inch round baking pan.
In a medium-sized mixing bowl, beat together the butter and sugar on medium speed, scraping down the sides of the bowl until very smooth. Reduce the mixer speed to medium-low and add the eggs, 1 at a time until the eggs are fully incorporated. Scrape down the sides of the bowl, then add the vanilla, 1 tsp of lemon zest, 1/4 cup of the lavender and Creme Fraiche.
In a separate bowl, sift together the flour, baking powder, baking soda, and salt. With the mixer on low speed, add the flour mixture to the batter until just combined.
Spoon the batter into the prepared pan and toss the blueberries with some of the lemon zest and lavender and sprinkle it over the batter.
For the streusel topping: combine the sugars and the spices, stir in the melted butter then the flour. Mix well. With your fingers, crumble the topping evenly over the batter and fruit.
Bake for 40 to 50 minutes or until a knife (or any cake tester) comes out clean. Cool completely and serve sprinkled with powdered sugar.
Sub Sour Cream for Creme Fraiche if that’s what you have available.