Step 1For the cake, preheat the oven to 177°C (350°F). Grease three 6in (15cm) cake pans, and line them with parchment paper. In a medium bowl, whisk together the flour, baking powder, salt, cardamom and cinnamon.
Step 2In the bowl of an electric mixer fitted with the whisk attachment, beat the butter, oil and granulated and brown sugars, on a medium speed, until they’re light and fluffy. This should take 2-3 minutes. Then add the eggs, one at a time, beating well after each addition for about 30 seconds. Make sure to scrape the sides and bottom of the bowl as needed. Add the sour cream and vanilla, and mix just until they’re combined.
Step 3With the mixer at low speed, add half of the dry ingredients to the mixture, followed by the milk and then the second half of the dry ingredients. Increase the mixer speed to medium, and beat the mixture, just until the flour is fully incorporated, making sure to scrape the flour from the sides of the bowl.
Step 4Distribute the batter evenly among the prepared pans, filling each pan only one-half to two-thirds full. Bake for about 35 minutes, until an indentation made with your finger in the top centre of the cake springs back. If the indentation remains, return the cakes to the oven for 2 minutes, or until the centre springs back. Remove the cakes from the pans, and allow them to cool completely before frosting.
Step 5For the blueberry buttercream, make the batch of Swiss Meringue Buttercream. Begin by thoroughly cleaning the aluminium bowl and the whisk attachment of the stand mixer; they can’t have any fat residue when making meringue. Once they’re clean and dry, place the egg whites and sugar in the bowl.
Step 6In small saucepan, bring 120ml of water to a simmer. Keep the water simmering on low heat, and place the bowl with the egg whites and sugar on top of the saucepan, creating a double boiler. Using the handheld whisk, whisk the egg whites and sugar over low heat for 5-10 minutes, until the sugar granules are dissolved. The sugar is fully dissolved when you don’t feel any grains of sugar when you rub the mixture between your fingers. If you use a candy thermometer to test for doneness, bring the mixture to about 65°C.
Step 7Remove the bowl from the heat, place it on the stand mixer and beat the egg whites with the sugar at high speed, until stiff peaks form and the mixture is no longer warm. This usually takes 7-10 minutes. Lower the speed to medium, and slowly add the butter, 2-3 tbsp at a time. Once all of the butter is incorporated, continue beating the mixture on medium-high speed for a few minutes, or until it emulsifies into a buttercream. The mixture may go through phases of looking soupy and curdled before it emulsifies. Scrape down the sides of the bowl, add the salt and mix just until it’s incorporated.
Step 8Add the vanilla bean paste and blueberry preserves and mix until they’re fully incorporated.
Step 9Place the first layer of cake on a flat surface and use an offset spatula with half of the frosting to frost the first and then the second layer of cake. Place the third layer on top, then smooth out any buttercream that squeezed out between the layers. Apply a thin crumb coat to the outside of the cake, and refrigerate the cake for at least 30 minutes.
Step 10Place the rest of the buttercream on top of the cake, and work it down the sides with an offset spatula. Use a cake scraper to smooth out the top and sides of the cake as needed.
Step 11This cake is best served at room temperature. Store it in an airtight container in the fridge for up to a week.