Preheat your grill to 365°F. Prepare to use the indirect grilling method. Prepare 3 cake pans with cooking spray and flour, knocking out any excess flour. Set aside
Cut the butter into cubes and then using a stand mixer, beat it until smooth and creamy. Add the sugars, creaming them into the butter until they are light and fluffy – about 3 minutes, the butter and sugars will become a light yellow color. Add the eggs, one at a time, and then the vanilla.
Sift together the flour, baking powder and salt. Slowly add the flour to the butter mixture. Then add the lemon juice and milk, mixing on low speed until just incorporated.
Toss the blueberries in the flour to coat, then fold them and 1/3 of the lemon zest into the batter.
Divide the batter evenly between the prepared cake pans. Bake for 35 minutes, or until the cakes are golden, edges coming away from the pan a little, and a toothpick comes out clean/with a couple crumbs.
Allow the cakes to cool a little in pan before removing them to a wire rack to cool completely. You can also freeze them for easier icing.
While the cake is cooling, it’s time to make the icing. Cut the butter and cream cheese into cubes. Beat on medium high until they are fully combined. Add the vanilla extract and optional salt, then slowly add the confectioners sugar, being careful because it will go everywhere if the mixer is going too fast. If the icing is too thick, add up to ¼ cup of cream, one tablespoon at a time until the right consistency has been reached.
To assemble, plop about 1/3 of a cup of icing onto the bottom layer of the cake. Spread it out evenly, it should be at least ½ an inch thick. Sprinkle with some of the remaining lemon zest, and extra blueberries if you want. Place another cake layer on top of that and repeat. Place the last layer on top of that, ice it and the sides of the cake with the remaining icing. Garnish the cake with lemon slices, blueberries, and remaining lemon zest.