Preheat the oven to 350 degrees F. Prepare a 9 inch round sponge cake pan by greasing the side with butter.
In a small pan melt the butter (4 tbsp) add the sugar, lemon zest and lemon juice.
Heat the mixture until it starts to bubble up, cook for about 2 minutes and then pour it into the prepared pan and swirl to evenly coat the base of the pan.
Add the blueberries to the pan gently pressing them down with the palm of your hand and covering the base.
Add the flour, baking powder and salt to a bowl and whisk to mix. Set aside.
In the bowl of a stand mixer, beat the softened stick of butter with the sugar until light and no longer grainy, scraping down as needed.
Add the lemon zest, vanilla and lemon oil and beat until well mixed.
Add the eggs one at a time scraping down and mixing well before adding the next egg.
Pour in half of the flour mixture to the batter and mix on low until incorporated.
Add the rest of the flour and mix.
Using a spatula transfer the cake batter to the pan and spread evenly over the blueberries.
Bake for about 45-50 minutes and the sponge is firm to the touch.
Let the cake rest for 10 minutes before placing a plate large enough to hold the cake, on top of the sponge.
Carefully lift the cake and plate and flip them over. Slowly lift the cake pan upwards and the cake should stay behind on the plate.