1. Line a 9-in-by-5-in loaf pan with plastic wrap. Set aside.
2. In a food processor or blender, puree the strawberries, stopping the machine when they are still a bit chunky. Add the granulated sugar and stir to combine. Place in the refrigerator.
3. In the bowl of a stand mixer, fitted with the whisk attachment, add the heavy cream, vanilla, and confectioners' sugar and whisk on medium to medium-high speed until the cream holds stiff peaks. With icebox cakes, I like to whip the cream a tad bit longer than one might otherwise do, as a stiffer cream ensures a sturdy cake.
4. Divide the cream into thirds, leaving one third in the stand mixer bowl and placing the other two in separate mixing bowls. To one of the mixing bowls, add the puree and fold gently with a rubber spatula. I like to add a drop or two of red food coloring, as well, so the strawberry color really pops, but this is optional. Set aside.
5. Sift the cocoa powder over the cream still in the stand mixer bowl, return the bowl to the stand mixer, and whisk on medium until the cocoa powder is fully incorporated. Set aside.
6. Begin assembling the cake by placing a layer of strawberry whipped cream on the bottom of the prepared pan. This is easier said than done, as the plastic wrap makes it a bit challenging. Don't worry if it's not perfect. I like to make my strawberry layers just slightly bigger than the other layers, but that is just me.
7. Place a layer of graham crackers on top of the strawberry cream, breaking up additional crackers to fill in all the gaps. Ideally, you want a solid layer of crackers.
8. Layer the chocolate whipped cream over the crackers, then add another layer of crackers. Repeat with the vanilla whipped cream and a layer of crackers. Continue layering in this order (strawberry, chocolate, vanilla) once more. You should have 6 layers of whipped cream total, and six of crackers. The final layer will be one of crackers and may reach above the edge of the pan. You will have a bit of leftover strawberry whipped cream - hardly a tragedy.
9. Wrap the pan in plastic wrap and place in the refrigerator for 6 to 8 hours. When ready to serve, place the pan upside down on a serving platter and remove the pan and plastic wrap. Dust with cocoa (optional), & serve with fresh strawberries.