Add milk, custard powder, and cornstarch to a medium sized pot and whisk well until there are no clumps remaining. Add the sugar and salted egg yolk and whisk again. There may be some small chunks of salted egg yolk remaining, but this is okay.

Add the (regular) egg yolk, vanilla extract, and salt, and whisk once more to combine.

Move the pot to the stove and heat on medium, mixing with a flexible spatula constantly so the bottom doesn’t stick or burn, until the mixture thickens. Once the mixture is thick and you see bubbles emerging at the very center of the pot, turn the heat down slightly, keep stirring, and set a timer for 2 minutes. Continue cooking until the 2 minutes is up (this ensures the starch is completely cooked), then remove from heat.

Stir in the cold butter until it’s melted & fully combined, then transfer the mixture to a small bowl. Press a piece of plastic wrap directly against the surface and refrigerate for at least an hour. This custard can be made ahead of time and kept in the refrigerator for up to 3 days until ready to use.

When using very cold custard filling, it will firm up quite a bit. Don’t worry, just “smush” and stir it with a spoon or spatula, and it will loosen and return to a velvety texture.

Add butterm sugar, salt, and baking powder to the bowl of a food processor. Blitz until the butter is no longer stiff, but is still cold (don’t over mix at this step).

Add the custard powder and flour and process until the mixture turns to the texture of a fairly uniform, fine gravel.

Add the egg yolks and process one more time until you can see the yellow of the yolks fully incorporate.

Dump the crumbly mixture onto a large piece of plastic wrap. Gently press and knead the mixture to form a crumbly, but cohesive, log. Wrap tightly in plastic wrap and press/roll the log once more to even it out before transferring it to the fridge to chill at least 2 hours. This can be made ahead of time and kept in the fridge for up to 3 days before using. Mixture should also freeze well, though I haven’t done any testing on this.

Preheat oven to 350F.

Slice croissants in half (like a sandwich). Fill each “sandwich” with about 2 Tbsp of custard filling and gently press with the back of a spoon (or clean hands) to level it out a bit. Replace the top of each croissant.

Divide the topping mixture into 6 even pieces. Starting with one piece, quickly and firmly press and knead the mixture in your hand until it softens into a pliable dough (though you still want it to be firm/cool to the touch, not melting or sticky). Roll the dough between your hands to form an oblong cylinder shape, about 2-3 inches long.

On a clean work surface, use the heel of your hand to press the oblong dough cylinder into a shaggy “rectangle” shape (it will be far from a true rectangle, but you want it to be a shape that’s somewhat long and narrow). If the topping becomes really sticky at any point, dust it lightly with some flour.

Use a bench scraper (or a sturdy metal spatula, or something similar) to scrape the blob shape off your work surface and slide it onto the top of a croissant. No need to press the topping down.

Repeat this process until all 6 croissants are topped.

Make the egg wash by combining the egg yolks, water, and salt in a small dish. Brush the the topping of each croissant with this egg wash. You don’t want to brush the croissant itself, only the bolo topping.

If desired, use a small knife to very lightly score the surface of each bolo piece in a grid shape. The idea is not to cut all the way through the topping — you’re really only trying to score the egg wash.

Bake croissants for 15-20 minutes until the topping is lightly puffed and golden brown. If desired, after 15 minutes, you can move the croissants under a broiler (only do this if your broiler is on top of your oven, not the bottom) to achieve a darker color. If you do this, be careful to keep a very close eye on the croissants so they don’t burn, which can happen very quickly.

Remove from oven and let cool slightly before enjoying. These are best enjoyed the day they’re made.