In a medium saucepan, whisk together the egg yolks, sugar, and blood orange juice. Place the saucepan over low heat (around a 4 on a scale of 1-10). Stir constantly for about 5 minutes before adding the butter. Continue to cook, stirring, for another 7 minutes, until the curd thickens slightly. Remove from heat, strain the curd to remove any lumps, and stir in the heavy cream. Cool completely in the refrigerator for several hours.

Preheat your oven to 350°F (175°C). Grease and flour your cake pans or use non-stick spray. Beat the softened butter and granulated sugar in a mixer until light and fluffy. Add the eggs, egg yolk, vanilla extract, orange extract, and sour cream. Mix until smooth. Add the flour, baking powder, and salt, followed by the milk and blood orange juice. Mix on low until well combined. Pour the batter into the pans and bake for 25-30 minutes, or until a toothpick comes out clean.

Let the cakes cool in the pans for 15 minutes. Then, carefully flip them out onto a wire rack and allow them to cool completely. Freeze the cakes for 20 minutes to make frosting easier.

Beat the softened butter in a mixer for about 30 seconds. Gradually add the powdered sugar, followed by the blood orange juice and orange extract. Mix until smooth and fluffy.

Place one layer of cake on a serving plate and pipe a ring of buttercream around the edges to create a barrier. Spoon half of the blood orange curd into the center and smooth it out. Add the second cake layer, then repeat with the remaining curd. Top with the final cake layer, inverted to create a flat surface. Frost the entire cake with the buttercream and decorate as desired. Refrigerate leftovers.