Grease and flour two 8-inch round cake pans (with high sides), lining the bottoms with parchment paper rounds for easy release. Preheat your oven at 325 degrees Fahrenheit (165 degrees Celsius).
In the bowl of your stand mixer (or in a large bowl using a hand mixer) whip the butter on medium high speed until very smooth. Add the sugar and mix on high speed until fluffy and creamy. The sugar will start to dissolve into the butter during this creaming process, so you should be able to feel the grains of sugar getting smaller and less noticeable.
Add the eggs one at a time, mixing well after each addition. Scrape down the sides of the bowl regularly at this stage to make sure everything is mixed well.
Mix in the zest of 2 lemons and vanilla extract on low speed.
Measure the milk into a liquid measuring cup and add the juice of one lemon to it. Mix and set aside.
Measure and sift the flour, baking powder and salt into a separate bowl and combine with a wire whisk.
Add the dry ingredients and the buttermilk alternatively while the mixer is running on low speed. Add 1/3 of the dry ingredients, half the milk, another 1/3 of the dry ingredients, the other half of the milk, and the remaining dry ingredients, leaving about 2-3 tablespoons of the flour aside to toss with the berries.
Add the blueberries to the remaining flour and toss well. Fold them into the cake mixture gently with a spatula.
Pour the batter into the prepared cake pans, dividing the batter evenly between the two pans. Bake at 325 degrees Fahrenheit (165 degrees Celsius) for about 40-45 minutes, or until a toothpick inserted into the middle of one of the cakes comes out clean.
Cool the cakes in the pans for about 10-15 minutes and then turn the cakes out of their pans and onto a cooling rack, allowing them to cool completely.
Once the cakes are cool, store them wrapped in plastic wrap in the fridge until ready to frost.