1-First Step: Preheat the oven to 350°F (175°C). Grease and flour a 9-inch cake pan or line it with parchment paper.

2-Second Step: In a large mixing bowl, combine the cake flour, baking powder, and salt. Mix well to ensure even distribution.

3-Third Step: In another bowl, cream the softened salted butter and sugar until light and fluffy. Add the egg whites one at a time, mixing after each, then stir in the almond extract and milk.

4-Fourth Step: Gradually add the dry ingredients to the wet mixture, stirring gently until just combined. Avoid overmixing for a tender crumb.

5-Fifth Step: For the filling, mix cornstarch, cold water, lemon juice, blackberries, and sugar in a saucepan. Cook over medium heat until thickened, then set aside to cool.

6-Sixth Step: Pour the batter into the prepared cake pan and smooth the top. If using, layer the filling in the middle for added flavor.

7-Seventh Step: Bake for 35-40 minutes or until a toothpick inserted into the center comes out clean.

8-Final Step: Let the cake cool in the pan for 10 minutes before transferring to a wire rack. Once cooled, frost with the mixture of fresh blackberries, lemon juice, butter, and powdered sugar. This recipe can be adjusted for dietary needs by using substitutes mentioned earlier, ensuring everyone can enjoy this delightful blackberry almond cake.