Preheat oven to 350°F / 175°C and grease two 8-inch pans
Mix dry ingredients together
Cream butter and sugar for 3–4 minutes
Add eggs, vanilla, and zest to the butter mixture
Alternate adding dry ingredients and buttermilk/orange juice mixture
Bake for 25–30 minutes and cool completely
Heat milk with half the sugar for custard
Whisk egg yolks with remaining sugar and cornstarch
Temper yolks with hot milk and cook until thick
Add butter, vanilla, orange juice, and zest to custard and chill until spreadable
Layer first cake layer, spread custard, add top cake layer
Whisk powdered sugar, orange juice, zest, and salt together for glaze
Drizzle glaze over cake and let it drip down the sides
Slice to reveal the custard layer
