GRILL: Preheat the grill to medium-high (about 400 degrees F) or heat a grill pan over high heat. Prepare the grill by cleaning with a grill brush to remove any built-up residue and then oil the grill. I do this by drenching a rolled-up paper towel in vegetable oil and, holding it with tongs, rubbing it liberally over the grill grates (or ridges in a grill pan). Oil the grill right before adding the pineapple.
PINEAPPLE: Remove the rind and stem from the pineapple and cut into 1/2 to 3/4-inch thick slices (you can also remove the core as desired; we remove it after grilling). Rub brown sugar over the slices and let stand until the grill is fully heated. Place pineapple slices on the grill and cook, flipping once, until tender, golden and lightly charred, about 2-4 minutes per side (this may vary, depending on the thickness, ripeness of pineapple, and actual heat of grill). Use a wide metal (sharp) spatula/turner for easy removal and flipping of the pineapple slices. If you find the pineapple sticking at all, use a bit more oil on the grill grates.
CRUMBLE: Preheat oven to 350 degrees F. Spray an 8x8-inch baking pan with cooking spray or line with parchment paper and set aside. Finely chop 1/3 cup of the walnuts and add to a large bowl. Crush the graham crackers into crumbs (using a blender or food processor or crush in a bag using a rolling pin) and add to the bowl. Add the brown sugar, white sugar, melted butter, cinnamon, salt, and vanilla to the bowl and stir until combined. Pour the mixture into the prepared pan and spread evenly into one layer. Bake for 7 to 10 minutes or until lightly browned. Allow to cool and then crumble and sprinkle on top of frozen vanilla yogurt. Coarsely chop the remaining few tablespoons of walnuts and add on top of the crumble.
ASSEMBLE: Place grilled pineapple slices on plates and top each slice with a large scoop of frozen vanilla yogurt. Add the crumble on top and coarsely chopped walnuts. Enjoy immediately.