In a saucepan, combine cranberries, 1 cup sugar, orange juice, water, zest, and salt. Bring to a simmer, mashing the berries until they burst. Strain this mixture through a sieve into a bowl, discarding the solids.

In a separate bowl, whisk together the eggs and egg yolks, then gradually pour the warm cranberry mixture into the eggs, stirring continuously. Return it to the saucepan and cook over low heat until thickened. Once thick, remove from heat and add butter and flour, whisking until smooth. Cool completely.

Preheat your oven to 350°F (175°C). Grease and flour your cake pans. In a large bowl, beat together the unsalted butter and 2 cups sugar until light and fluffy. Add in eggs, buttermilk, orange juice, and vanilla, mixing until well combined.

In another bowl, whisk together cake flour, baking powder, baking soda, and salt. Gradually add this dry mixture to the wet mixture, stirring until just combined.

Pour the batter evenly into prepared pans and bake for about 25-30 minutes or until a toothpick inserted in the center comes out clean. Allow the cakes to cool completely in the pans before removing.

Once cooled, spread the cranberry curd between the layers of cake and frost with a vanilla Swiss meringue buttercream.