Place cooled cake on a cake turntable. Gently score around a line at half the height of the cake with a bread knife
Keep turning the cake and gradually cut into the cake along the scored line. This should yield a bottom cake layer and a top cake layer
Carefully lift the top layer using the bread knife and aside on a separate plate
Using a spatula, spread a 1/4 inch payer of icing over the bottom cake layer
Place berries (blueberries in my case) as tightly packed and as evenly distributed over the icing as you can. Gently press the berries into the icing to secure position
Spread a thin layer of icing over just enough to cover the berries, then place the top cake layer on top
With a spatula, take about 2 tbsp of icing at a time to put on the side of the cake and even it out by scraping the icing at an angle.
Continue to turn the cake and spread icing on the side until all the cake and fillings are concealed evenly
Spread icing on top of the cake and even it out with a spatula
Add berries and walnut pieces to decorate as desired
Bring the cake to the party and let your loved ones enjoy the surprise!