Preheat the oven to 350 degrees F.
Coat a 9" x 13" baking pan with the oil spray. Cut a 13" x 13" piece of parchment paper to fit the bottom and up 2 of the 4 sides of the pan. (The parchment paper will later be used as handles to lift the brownie out of the pan.)
Sift the flour, cocoa and baking powder into a bowl. Mix in the nuts. Place the sugar, butter and vanilla in a second bowl and beat to combine. Beat in the eggs. Mix dry ingredients into the wet until just combined. Spread batter into the pan. Bake for 25 minutes.
Cool brownie to room temperature. Run a knife around the outer edges of the pan. Grab the parchment above the brownie and carefully lift it out of the pan and onto a work surface. Carefully remove the parchment from the brownie and set on a serving plate.
Place the jelly in a small pot and melt over medium heat. Spread the jelly on top of the brownie. Let cool a few minutes, and then arrange the sliced strawberries on the brownie. Refrigerate until needed. When ready to serve, dust with icing sugar, and then cut the brownie into bars.