Combine all ingredients EXCEPT the water into a large bowl. Mix well, then add 3/4 cup cold water. Mix with a fork until a shaggy dough, then turn out onto a clean surface to knead until a very soft and pliable dough. Add up to 1/4 cup more water, as needed.

Cover and let rest at room temperature for 15 minutes.

Roll out the dough until 1/8" thick, about an 18" square.

Divide the cubed butter into thirds, about 5-6 tbsp per third.

Spread one third of the butter on two-thirds on the dough. Fold like a letter, by folding the unbuttered dough side over the center. Then, fold the remaining flap over the center, as well. Watch the video for a better visual guide.

Making sure the surface stays well-floured, roll out the dough to a very thin rectangle again. Brush with another third of the softened butter and fold same as last time.

For the final time, roll out the dough to 18x21" rectangle. Spread all over with the remaining third of butter. Roll up tightly width-wise into a log. Cover and let chill in the fridge 1-2 hours.

Grease 1-2 muffins tins.

Slice the chilled dough log into 1" pieces, then semi-flatten in the palm of your hands. Use cold water to help it not stick, if needed.

Place the flatten dough into the greased muffin tin and use your thumb to press it up along the muffin tin edges. You're creating a tart cup. Repeat with remaining dough.

Preheat oven to 500F for at least 20 minutes.

In a large bowl, whisk the flour and 1/4 cup milk until smooth. Set aside.

In a saucepan, combine the sugar, cinnamon, and remaining water. Place over medium high heat to boil until 220F. Remove from heat.

Whisk as you gradually pour the sugar mixture into the flour-water bowl.

In the same saucepan, scorch the milk by bringing to a boil.

Gradually whisk the hot milk into the bowl.

Add one egg yolk at a time, whisk the yolks into the bowl until smooth.

Fill each tart cup 3/4ths full with the custard mix.

Bake for 8-10 minutes, until tops are deeply golden.

Let cool at least 30 minutes, then dust with powdered sugar to enjoy!