To begin, make the creme patisserie. Combine milk, sugar #1 and vanilla seeds in a small pot and bring to a boil.

In a separate bowl, add the cornstarch and sugar #2 into a bowl and mix to combine. Add the egg yolks to the mixture and whisk until smooth.

Once the milk boils, pour 1/3 milk over the sugar mixture and whisk.  Add the remaining 2/3 milk and whisk. Strain the mixture into a clean pot and return to the heat.

Continue to cook the mixture, stirring constantly until it has boiled for 2 minutes and has completely thickened. Remove the pot from the heat and stir in the butter.

Pour the crème patisserie into a container and cover the surface with plastic wrap, pressing the plastic onto the surface so a skin doesn't form and place in the refrigerator to cool.

When the crème patisserie is cool, transfer to the bowl of a stand mixer fitted with the paddle attachment. Beat for one minute to soften the creme patisserie.

Add in the soft butter and icing sugar in small amounts and continue to beat until light and fluffy. Add in the flavouring of choice and mix until incorporated and smooth. Refrigerate until use.

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