Preheat the oven to 350F/177C. Line three, 8-inch (20-cm) cake pans with parchment paper and spray the sides with a baking spray.
In a large mixing bowl, combine the eggs, buttermilk, pure maple syrup, cooking oil and vanilla extract. Beat on medium speed for a couple minutes, until the eggs are well-beaten.
In a separate mixing bowl, combine the dry ingredients: all-purpose flour, almond flour, baking powder, baking soda and salt. Sift the dry ingredients into the cake batter and beat on medium speed for about 1 minute, just until the dry ingredients are incorporated.
Divide the cake batter evenly between the three pans. Bake in the preheated oven for 23to 25 minutes, or until a toothpick inserted into the center comes out clean. Remove the cake layers from the oven, out of the pans and onto a wire rack to cool completely.
While the cake layers are baking and cooling, prepare the berry filling. Combine the diced strawberries, raspberries and blueberries in a large mixing bowl. Pour the pure maple syrup over the berries and let them stand at room temperature until ready to use.
Next, prepare the maple whipped cream frosting. Place the softened mascarpone cheese into a large mixing bowl and add the pure maple syrup and vanilla extract. Whisk on medium speed for 1 minute, or until the mixture is smooth and creamy. Pour in the chilled heavy cream next and whisk on medium-high speed for 4 to 5 minutes, until stiff peaks form. Store the frosting in the refrigerator until ready to use.
To assemble the cake, first use a long, serrated knife to level off the top of each cake layer. Spread a generous amount of whipped cream onto each cake layer. Transfer about 1 ½ cups of the whipped cream frosting into a pastry bag tipped with a star tip, such as the French star tip #8 that I used. Pipe a barrier ring around the edge of the cake layer; this will keep the berries inside.
Add about 1/3 of the maple berries into the center, then top with the next cake layer and repeat the same process. Frost the top and sides of the cake with the remaining whipped cream frosting. Place the cake into the refrigerator to chill for about 20 minutes before garnishing the top.
Transfer any remaining frosting into a pastry bag tipped with a star tip; I used a French star tip #4. Add dollops of frosting around the rim of the cake, then add the remaining berries into the center. Garnish the edges with fresh strawberries, if desired.
For best results, refrigerate the cake for at least 4 to 6 hours or overnight before serving. Store the cake in the refrigerator when not enjoying.