Heat the milk in a small pot.
In a medium bowl mix sugar and cornstarch. Add the egg yolks and mix with a whisk.
When the milk starts to boil pour half of it in the bowl using a strainer. Mix to make the solids dissolve. Add the remaining milk and stir.
Put everything back in the pot on medium heat, stirring constantly with the whisk until the cream thickens.
Remove from heat, transfer in a bowl (preferably cold) and whisk vigorously for a minute or two.
Spread the custard cream on a baking tray lined with cling film. Cover the top with cling film and let it cool off.
In the stand mixer with paddle attachment work the butter at medium speed. Add 2 tbsp of custard cream (it should not be hot!) and the 110g (3.9 oz) of pecan nut praliné.
Let the mixer work at medium speed to combine all ingredients.
Add the remaining custard cream and work until the mixture becomes uniform.
Transfer to a bowl, cover with cling film and store it in the fridge.
The custard cream can also be transferred directly in a pastry bag with round tip (which will be used later on). The problem is that if it remains in the bag for too long it will set and will have to be worked again before use.