To make buttercream:
Place flour and sugar in saucepan and whisk to combine. Add milk and cook over medium heat, stirring constantly to boiling. Continue to cook for 45 seconds. Remove from heat, pour into a bowl and cover with plastic wrap directly onto buttercream base and place in refrigerator until cold (overnight is best).
Place butter in bowl of stand mixer and whip until fluffy and lighter in color. Add spoonfuls of Buttercream base, thoroughly incorporating between additions. Add pinch of salt and vanilla and mix for another minute until combined. Set aside in a cool place until ready to frost.
To make raspberry purèe:
Thaw raspberries. With a spoon, mash berries to a fine pulp. In a small saucepan, combine raspberry pulp, sugar, lemon juice and cornstarch. Cook over medium heat, whisking constantly until mixture bubbles and thickens. Remove from heat and set aside to cool. Once cooled, add ½ pint of fresh raspberries to mixture and set aside.
To make cake:
Preheat oven to 350 degrees and prepare two 8" round cake pans by lining the bottoms with a round of parchment paper and generously greasing and flouring the sides. Be sure to shake out excess flour.
In a stand mixer, beat butter on medium-low speed until creamy. Add sugar and oil and beat until all ingredients are well combined and creamy. Scrape down the sides and bottom of the bowl and then stir in your vanilla extract.
In a separate bowl, whisk together your flour, baking powder and salt. Measure out your milk. With mixer on medium speed, gradually alternate between adding the flour mixture and the milk, starting and ending with the flour mixture. Stir until each on is almost completely combined before adding next. Pause occasionally to scrape down sides and bottom of the bowl.
In a separate bowl, combine your egg whites, and with hand mixer on high speed, beat until stiff peaks form. Using a spatula, gently fold in your egg whites into your batter. Take care to scrape the sides and bottom of the bowl so that the ingredients are well-combined, and take care not to over-mix.
Evenly divide cake batter into prepared pans. Bake at 350 degrees for 35–40 minutes, or until a toothpick inserted in the center of each comes out clean. Let rest in pan for 5 minutes and invert them onto a cooling rack until completely cool.
Lay first layer cake on platter, and top with raspberry puree. Spread puree onto first cake, leaving 1 inch from the outside of cake. Top with 2nd cake layer. Starting from the top, place a large spoonful of icing onto top working to edge and down side of cake. Top with fresh strawberries and blueberries, plus fresh mint for garnish. Refrigerate until ready to serve.
Recipe courtesy of Jackie Collier, Retail Operations Project Manager at Simon Pearce