In a small bowl combine cocoa and boiling water, stirring until smooth. Set aside until completely cooled.
Preheat oven to 350ºF. Grease and flour three 9” layer cake pans.
In a medium bowl sift flour, soda, salt and baking powder.
In the bowl of an electric mixer, at high speed, beat butter, sugar, eggs and vanilla, scraping down sides of bowl occasionally, until light and fluffy – about 5 minutes.
At low speed, beat in flour mixture in four additions, alternating with cocoa mixture, starting and ending with flour mixture. DO NOT OVERBEAT.
Divide evenly into prepared pans and smooth the tops. Bake for 25 to 30 minutes or until top springs back when pressed with fingertip. Cool in pans 5 minutes and then turn out to cool on racks.
To assemble: (SEE ICING OPTIONS BELOW) Place first layer on a serving plate. Spread 1/3 cup of blueberry jam on top. On the top of the second layer spread ½ cup of chosen icing, then flip it over and place on top of jam-toped layer. Repeat with third layer, then frost cake all over with chosen icing (I chose to do trendy 'naked' sides and pile the icing a mile high on the top). Top with fresh blueberries.