Makes 12 babas.
Cook's Note:If neither of these are available to you, you can bake these in a mini muffin tin.
For the curd, whisk the butter, sugar and lemon zest in a small saucepan over medium heat until the butter has melted. Whisk the egg, lemon juice and cornstarch together in a bowl and add this to the pot. Keep whisking on medium heat until the curd thickens and begins to bubble, about 8 minutes. Remove the pan from the heat and strain the curd into a bowl. Place a piece of plastic wrap directly on the surface of the curd, cool to room temperature and then chill until needed.
For the babas, stir the flour, sugar and yeast together in a mixing bowl and add the water and egg. Stir with a wooden spoon or spatula until the dough is stretchy (it will be very sticky). Really work the dough with your spoon, stretching it and slapping it against the side of the bowl, to make it even more elastic. Thinly slice the butter and place this on top of the dough in a single layer, cover the bowl with plastic wrap and set on the counter for 30 minutes, until the dough almost doubles in size.
Lightly grease a silicone cannele pan* or dariole pan* (see Cook’s Note) and place this on a baking tray. Knock down the dough with a wooden spoon and stir well, working in all of the butter. Spoon the dough (still very sticky) into a piping bag fitted with a large plain tip and pipe the dough into each cup, filling up halfway. Set this aside, uncovered, for 30 minutes to rise.
Preheat the oven to 375ºF (190ºF). Bake the babas for 18-20 minutes, until golden brown on top. Let the babas cool for 5 minutes, then remove them from the tin to cool completely. These can be made and baked ahead of time, and kept frozen or in an airtight container until ready to assemble.
For the syrup, bring the sugar and water up to a full simmer over high heat, stirring occasionally and cooking just until the sugar has dissolved. Remove the pan from the heat and stir in the lemon juice.
To assemble, fill a piping bag fitted with a small plain tip (or doughnut tip) with the lemon curd. Dip each baba into the syrup using tongs and let the syrup soak in all the way through (the baba won’t fall apart) but then give it a gentle squeeze so that it is moist but not soggy. Insert the piping tip at the bottom of the baba and pipe in enough lemon curd until it starts to peek out. Set the baba onto a tray or in a paper cup and chill until ready to serve.