Preheat oven to 350°F (180°C). Lightly grease two 8-inch round cake pans and line bottoms with parchment paper. In mixing bowl, combine cake mix, water, oil and eggs. Using electric beaters, beat on low speed for 30 seconds to moisten. Scrape down side of bowl. Beat on medium speed for 1 minute or until smooth. Divide batter evenly between prepared cake pans. Bake cakes in centre of oven for 25 to 30 minutes or until a toothpick inserted in centre comes out clean. Remove from oven and let cool for 10 minutes before removing from cake pans. Transfer to wire rack to cool completely before icing, about 1 hour. Meanwhile, place frozen raspberries in saucepan. Cook over medium-low heat for 5 minutes, stirring
constantly, or until berries are thawed and come to a simmer. Stir in 1⁄2 cup (125 mL) icing sugar; cook, stirring, for 10 to 15 minutes or until mixture thickens and reduces. Set fine-meshed sieve over bowl. Pour raspberry mixture into sieve; using rubber spatula, press purée through sieve. Discard seeds in sieve. Refrigerate raspberry purée uncovered until cold, about 30 minutes, stirring once or twice. In stand mixer fitted with paddle attachment, beat butter, salt and 2 cups (500 mL) of the icing sugar on low speed until combined. One cup at a time, beat in remaining icing sugar; continue to beat until light and fluffy, about 2 to 4 minutes. Beat in 2 tbsp (25 mL) of chilled raspberry purée. Using serrated knife, split each cake horizontally into two even layers. Carefully slice top of one of rounded layers to make it level; set the remaining rounded cake layer aside. Place one of level cake layers bottom side down on cake plate. Spread with 2 tbsp (25 mL) of remaining raspberry purée. Top with 3⁄4 cup (175 mL) raspberry buttercream, spreading just to edge of cake. Repeat cake layer, raspberry purée, raspberry buttercream layers twice. Top with rounded cake layer, top side up. Spread with 2 tbsp (25 mL) raspberry purée; swirl or pipe remaining raspberry buttercream on top, leaving side of layer cake unfrosted. Discard remaining raspberry purée or save for another use. Garnish cake with fresh raspberries, if desired. Makes 12 servings.