In a large bowl, combine flour, cocoa powder, salt, and sugar.

Cut 1 stick of butter into small pieces and work into the flour mixture with your hands or a fork until you achieve a sandy texture.

Add 1 egg yolk and mix until a dough forms. Don't overwork.

Form dough into a disc shape. Wrap dough in cling film and transfer to refrigerator to chill for 30 minutes - 1 hour.

Remove tart dough from fridge and let warm for 10-15 minutes.

Roll out the dough on a floured work surface into a circle slightly larger than the size of your tart pan.

Transfer tart dough to your tart pan and press the dough into the bottom and sides to adhere. If you have any cracks or loose bits you can patch them in easily.

Once you're happy with your dough (it doesn't need to be perfect) transfer to the fridge again to chill for at least 30 minutes.

Preheat oven to 350°F. Once oven is ready, remove tart shell from fridge and bake for 30 minutes.

Remove tart shell from oven and let cool slightly. In the meantime, chop up your chocolate and add to a large heat-proof bowl.

In a saucepan, heat heavy cream, milk, and 2 tbsp butter over medium until bubbly. Pour over chocolate and allow to sit for 1-2 minutes before whisking into a ganache.

To your ganache, add 2 egg yolks, one at a time, whisking vigorously to avoid scrambling. Add a pinch of sea salt and make sure there are no lumps or unmelted chocolate bits.

Place tart shell on a sturdy baking sheet. Pour ganache mixture into tart shell and bake for another 10-15 minutes until the filling is slightly wobbly but mostly set.

Remove tart from oven and chill for 2-3 hours on your countertop. The chocolate will begin to set, but be careful not to touch the top.

Once the tart is cool, transfer to the fridge for 1-2 hours until firm. After that you can wrap in cling film without damaging the top and either freeze the tart for another day or continue to set in the fridge until you are ready to serve.

When ready to serve, carefully remove the tart from the tart pan and top with raspberries and blackberries.

You can optionally brush the berries with a simple syrup for a beautiful gloss and garnish with edible flowers.