Heat oven to 350 degrees. To make the brownies, prepare a 8 inch square pan with parchment or butter. In a double boiler, melt butter with 3/4 cup chocolate chips, set aside to cool. In a bowl, mix flour, cocoa, baking powder together. In a separate bowl, whisk eggs together, add vanilla. Next add melted chocolate mixture and finally add in the dry ingredients and remaining 1/4 cup chocolate chips. Pour into pan and bake for 20 minutes or until set. Let cool in pan. Cut into small 1 inch cubes for trifle.
To prepare the mousse, heat 3/4 cup cream until almost boiling. Pour over 1 ½ cups chocolate chips in a bowl and let sit a few minutes before stirring. Stir until smooth, let cool. In a medium bowl, whip 2 1/4 cups cold cream with 2 T powdered sugar and vanilla until soft peaks form. Gently fold in cooled chocolate cream. Whisk again until soft peaks form. Cover and refrigerate for 1 hour.
For the trifle layering, whip the 2 cups cream with 2T powdered sugar and 1 tsp vanilla until soft peaks form.
In a trifle dish, begin layering. First, brownies on bottom, next chocolate mousse, then whipped cream and berries. Repeat until ingredients are gone or desired height and look. Top with berries. Refrigerate 1-2 hours then serve. Serves 6-10.