Cut cold butter into 1 inch cubes.
Strain the egg yolks through a sieve. Beat with a french whisk.
While mixing, add in the sugar. It’ll turn into a light lemon color.
Add a clamshell of blackberries to a plastic ziplock bag.
Using a rolling pin, mash the berries into a pulpy consistency.
Use a muddler and a sieve to strain any juice through.
Measure out the juice to be about ⅓ cup or 80ml juice.
Add the blackberry juice to the mixture.
Transfer the bowl to a medium low heat while whisking constantly.
Once it hits 170F, it will change consistency. This can be tested by dipping the back of a wooden spoon and running your finger through.
Add in the cubed butter until combined.
Pour into a bowl, cover with plastic and transfer to the fridge to set for a few hours or even overnight.
When set, transfer to a piping bag for assembly. Snip off the tip.