Line a 9-in. (23 cm) round baking pan with foil. Spread frozen yogourt in an even layer in pan; freeze for 2 hours, or overnight.
Preheat oven to 350ºF (180ºF). Spray or brush two 9-in. (23 cm) round baking pans with oil. In a large bowl, using electric mixer on low, blend cake mix, oil and eggs with 1 cup (250 mL) water until moistened, about 30 sec. Increase mixer to medium; beat for 2 min. Divide batter between prepared baking pans. Bake until toothpick inserted in centre of cake comes out clean, 24 to 29 min. Let cool in pans on rack.
In a dry skillet over medium-high heat, stir together walnuts and maple syrup. Cook, stirring frequently, until syrup has caramelized, 5 to 7 min. Spread walnuts in a single layer on a baking sheet lined with parchment paper; let cool completely.
Place 1 cake on a serving plate. Remove frozen yogourt layer from pan and place on top; sandwich with remaining cake. Using a palette knife, smooth edge of yogourt layer, filling in gaps between layers. Freeze cake for 1 hour.
In a large bowl, whip cream to stiff peaks. Remove cake from freezer. Spread whipped cream on top; garnish with strawberries and walnuts.
You can make candied walnuts in advance and store them in an airtight container at room temperature for up to 2 weeks.