Prepare the keto chocolate sponge cake first. Preheat the oven to 350F/177C and prepare three, 8-inch cake pans by lining the bottom with parchment paper; leave the sides ungreased. Place the eggs into a mixer bowl and whisk on high speed for 4 to 6 minutes, until the eggs are fluffy, voluminous and pale in color. Add the vanilla and coffee extracts and whisk again for 30 seconds.
In a separate bowl, combine the almond flour, erythritol, baking powder and cocoa powder. Sift the dry ingredients into the eggs in three small increments, folding gently but thoroughly after each addition. Watch my video recipe to see how it's done. Add in the coconut oil last and very gently fold it into the batter for about 30 seconds; it does not need to be mixed in thoroughly.
Divide the prepared cake batter evenly between the three pans; use a kitchen scale if desired. Use an offset spatula to spread the cake batter evenly to the edges. Bake the cake layers for 17 to 19 minutes, just until the cake batter is set; the top of the cake layers will be glossy and bubble. Remove the layers from the oven and onto a cooling rack to cool. After about 15 minutes, run a knife along the edges of the pans to release the cake layers, gently invert the cake, remove the parchment paper and allow to cool completely. The cake layers will be fragile; handle with care! If a layer splits, that's perfectly fine; the buttercream will keep it together!
For Strawberry Buttercream:
Once the layers are cooled, prepare the strawberry keto buttercream. Place the softened butter into a mixer bowl and whisk on high speed for 4 to 5 minutes, until the butter is light, fluffy and almost white in color. Scrape down the sides of the mixing bowl often to ensure that all the butter gets whisked.
Next, add in the powdered erythritol and mix it in on the lowest speed for about 1 minute, until incorporated. Once incorporated, turn the mixer up to high speed and continue whisking for another 2 to 3 minutes, until the buttercream is light and fluffy. Add the strawberry extract and sugar-free preserves last and whisk for up to 30 seconds, or until well combined. Transfer about 1 1/2 cups of the buttercream into a pastry bag tipped with a star tip; I used Ateco #847.
Assembling the Cake:
To assemble the cake, spread a medium amount of buttercream onto the first layer and use an offset spatula to spread the buttercream evenly. Pipe a barrier ring of frosting along the edge of the cake layer, then add in 1 cup of the diced strawberries; the barrier will help keep the strawberries inside. Press the strawberries gently into the buttercream to keep them in place.
Repeat the same process for the second cake layer and top with the last one. Frost the top and sides of the cake, spreading evenly with offset spatulas and cake spatulas. After applying a crumb coat, freeze or refrigerate the cake for at least 10 minutes, then add the final layer of frosting. Garnish the cake with dollops of the remaining buttercream and fresh strawberries. Keep the cake refrigerated for up to 3 days.