In a stand mixer fitted with the paddle attachment, cream the butter, shortening, sugar, vanilla, and almond extract on medium speed until light and fluffy, about 5 minutes. 

Turn the speed to medium-low and mix in the egg whites in 6 additions, mixing until smooth after each one. Wipe the sides of the bowl after each addition. 
In a separate bowl, whisk together the flour, baking powder, and salt. Sift to remove any lumps.

Add 1/3 of the flour mixture to the butter/egg white mixture, mix until combined. Scrape bowl and add half the buttermilk, mixing until combined. Repeat with another 1/3 flour, the remaining buttermilk, and then the final 1/3 flour. Scrape after each addition. 

In another bowl, whip the heavy cream until medium peaks. Fold the whipped cream into the batter with a rubber spatula until there are no streaks of cream. Spread the batter into the prepared pan.

Bake for 20-25 minutes, until set. (the timing may take more or less time depending on the pan you use.)

Return the bowl to the stand mixer fitted with a clean whisk attachment and beat on medium-high speed until the egg foam is light, fluffy, and glossy and the bowl feels just about room temperature. (If the egg foam isn't cooled sufficiently, the butter will melt when you add it.)

Once the egg foam is whipped and cooled, turn the mier speed to medium, add the butter, 2 tablespoons at a time, and beat until incorporated. Turn the speed to low, add the vanilla and salt, and mix until incorporated. After the buttercream is whipped, add the passion fruit a little at a time until it is smooth.