For the Orange Almond Poppy Seed Cake:
Preheat the oven to 325°F (165°C). Grease and line three 8-inch round cake pans with parchment paper. Set aside.
In a medium bowl, whisk together cake flour, baking powder, orange zest, salt, and soaked poppy seeds. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, cream the butter and sugar on medium-high speed for 2 minutes until light and fluffy.
Gradually add the egg whites, one at a time, scraping down the sides of the bowl between additions. Mix in the almond extract. Continue beating on high speed for 2-3 minutes until the mixture becomes lighter in color and increases in volume.
With the mixer on low speed, alternate adding the flour mixture and buttermilk in three additions, starting and ending with the dry ingredients. Mix just until combined.
Gently fold in the sour cream.
Divide the batter evenly among the three prepared pans (about 16 oz per pan). Bake for 25-30 minutes or until a toothpick inserted into the center comes out clean. Cool in the pans for 15 minutes, then transfer the cakes to a wire rack to cool completely.
For the Almond Buttercream:
In the bowl of a stand mixer fitted with the paddle attachment, beat the slightly cold butter on medium-high speed until light and fluffy, about 2-3 minutes.
Gradually add the sifted powdered sugar, 1 cup at a time, mixing well after each addition.
Add the vanilla extract, almond extract, and whipping cream. Beat on medium-high speed for 3-5 minutes until smooth and fluffy.
Before using, mix the frosting by hand with a wooden spoon to release any air pockets.
For the Orange Curd:
In a medium saucepan, whisk together the sugar, orange juice, and orange zest over medium-low heat.
In a separate bowl, whisk together the eggs and egg yolks. Gradually add the eggs to the sugar mixture, whisking constantly.
Cook the mixture over medium-low heat, stirring constantly, until thickened like pudding (about 6-8 minutes).
Remove from heat and whisk in the cold butter, one piece at a time, until fully incorporated.
Strain the curd through a fine-mesh sieve into a small bowl. Cover with plastic wrap, pressing it directly onto the surface to prevent a skin from forming. Chill in the refrigerator for at least 4 hours or up to a week.
Assembly:
Place one cake layer on a cake plate or board, top side up. Spread about ½ cup of buttercream evenly over the top.
Pipe a rim of buttercream around the edge of the cake and fill the center with ⅓ to ½ cup orange curd.
Place the second cake layer on top and repeat with the buttercream and curd.
Add the third cake layer, top side down. Spread a thin layer of buttercream (crumb coat) over the entire cake and freeze for 15 minutes.
Once the crumb coat is set, apply the final layer of buttercream. For a decorative finish, use a Wilton 1M tip to pipe swirls around the cake.
Storage:
Store the cake in an airtight container in the refrigerator. Bring to room temperature for 2-3 hours before serving for the best texture.