Make the blackberry filling by adding fresh blackberries, sugar, and lemon juice to a saucepan over medium heat. Cook for 10 minutes, until the blackberries begin to become juicy and soften, then reduce heat to medium low and let the mixture simmer until the blackberries have softened completely, about 15-20 minutes.

Mix together cornstarch and water to create a slurry, pour into the pan and stir constantly until the mixture thickens into a jam-like consistency, about 5 minutes. Set aside to cool completely.

Preheat the oven to 325 degrees F. Spray 3 8-inch baking pans with nonstick baking spray. Set aside.

Add dry ingredients to a large bowl - all purpose flour, milk powder, baking powder, baking soda, and kosher salt. Mix together and set aside.

In the bowl of a stand mixer fitted with the paddle attachment, add lemon zest and sugar. Rub the zest into the sugar for about 5 minutes, until fragrant and the sugar begins to take on a pale yellow color. Add butter and vegetable oil and mix together on medium speed for 5 minutes, until pale and fluffy. Scrape down the sides of the bowl and reduce the speed to low speed. Begin adding the eggs and eggs whites, one at a time, ensuring each egg is fully incorporated prior to adding the next. Scrape down the sides of the bowl and add the vanilla extract, lemon extract, and almond extract. Mix on low until just combined, about 10-15 seconds.

In a small bowl, mix together buttermilk, lemon juice, water, and sour cream.

Add half of the flour mixture to the stand mixer ingredients and continue to mix on low until just combined, about 20 seconds. Then pour in the buttermilk mixture and continue mix until just combined, about 15 seconds. Scrape down the sides of the bowl and add the remainder of the dry ingredients.

Divide the cake batter equally among three 8 inch cake pans. Bake for 40-45 minutes on the center rack of the oven. Allow the cakes to cool in the pan for 5 minutes, then turn over onto a wire rack to cool completely.

While the cakes cool, make the blackberry frosting. Add the butter to the bowl of a stand mixer fitted with the paddle attachment. Mix on medium speed for 3 minutes, then add mascarpone and mix together for another 30 seconds. Then add 2 ½ cups of powdered sugar, mix until just combined, about 30 seconds. Add 2 tbsp of the cooled blackberry filling, almond extract, and the remainder of the powdered sugar. Mix until smooth and fluffy, about 1 minute.

Once the cakes have cooled completely, place the first layer onto a cake stand. Spread a thin layer of frosting on top of the cake. Add about a 1 cup of frosting to a piping bag. Pipe a border around the perimeter of the cake, fill with half of the blackberry jam filling. Place the second cake on top and repeat. Add the third layer and add a thin layer of frosting all over the entire cake. Place into the refrigerator for 30 minutes, then add the remainder of the frosting to the whole cake. Garnish with fresh blackberries or any fresh fruit you'd like.