Step 1 In a mixer, whisk together the egg yolks, sugar, cornstarch and vanilla seeds. Slowly add the buttermilk, whisking to combine.
Step 2 Place the bowl over a saucepan of simmering water and heat the mixture, whisking until it thickens.
Step 3Return the bowl to the mixer and mix until the mixture has cooled to lukewarm. Beat in the butter and Grand Marnier. Strain through a fine sieve. Place in a bowl, cover with plastic wrap and refrigerate until completely cooled.