Butter and line three 9 inch cake pans with parchment paper, or instead of lining use non stick cake pans and just butter well.
Combine the flour, cocoa, brown sugar, granulated sugar, baking soda and salt in a large mixing bowl; blend well, add the butter with the mixer on low, blend until crumbly.
Stir the milk with the lemon juice.
Add half the milk mixture, blending on low speed for 1 minute, increase the speed to medium. Beat in the remaining milk mixture, eggs, and vanilla until batter is smooth, scraping the sides of the bowl as needed.
Divide the batter evenly into the prepared pans. Bake for 25 minutes or until a toothpick inserted into the centre of one layer comes out clean. Cool in the pan for 10 minutes. Turn out onto a rack and cool completely.
In a clean bowl, beat the butter until creamy, beat in the eggs and vanilla until light; sift icing sugar with the cocoa; add the icing sugar mixture and beat until fluffy. Frost between the layers, adding a few berries on top of the frosting on each layer. Frost the sides and the top of the cake. In another bowl combine the frozen berries and the berry sugar, mix to coat.
Place half a dozen berries in the middle top of the cake on top of the frosting.
Serve the rest of the berries beside each slice of the cake.