Preheat your oven to 350°F (175°C) and grease the 9-inch round cake pan. For easier removal, line the bottom with parchment paper.
In a mixing bowl, combine the flour, baking powder, baking soda, and salt. Whisk together and set aside.
In a separate bowl, cream the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.
Add the eggs one at a time to the sugar mixture, mixing well after each addition. Stir in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, alternating with the milk. Begin and end with the flour mixture to ensure a smooth batter. Mix until just combined.
Gently fold in the dried lavender and fresh blueberries using a rubber spatula, being careful not to break the berries.
Pour the batter into the prepared cake pan and smooth the top.
Bake in the preheated oven for 30-35 minutes or until a toothpick inserted in the center comes out clean.
Once baked, remove the cake from the oven and let it cool in the pan for 10 minutes. Then, transfer it to a cooling rack to cool completely.