Preheat oven to 350°F / 175°C and grease two 8-inch pans
Mix dry ingredients (flour, baking powder, baking soda, salt)
Cream butter and sugar for 3–4 minutes
Add eggs, vanilla, and orange zest to the butter mixture
Alternate adding dry ingredients with buttermilk and orange juice mixture
Divide batter between prepared pans and bake for 25–30 minutes until a toothpick comes out clean
Cool cakes completely
For custard: Heat milk with half the sugar
Whisk egg yolks with remaining sugar and cornstarch
Temper the egg mixture by slowly adding hot milk while whisking
Cook the custard until thick, then add butter, vanilla, orange juice, and zest
Chill custard until spreadable
Layer one cake, spread custard filling, and top with second cake layer
For glaze: Whisk together powdered sugar, orange juice, orange zest, and salt
Drizzle glaze over cake and let it drip down the sides
Slice and serve to reveal the custard layer
