Add the all-purpose flour, powdered sugar, milk powder, butter, and milk to a food processor. Pulse the ingredients in long pulses (about 2 seconds each), until the mixture comes together and small curds start to form (about 10 to 13 pulses). Test the mixture by grabbing a clump of the curds and squeezing them together into your hand. If the curds come together into a lump of dough, you’re done mixing. If it still feels dry and crumbly, add a tiny splash of milk and keep pulsing the mixture.

Empty the dough curds into a mixing bowl. Gather the mixture together until you form a ball of dough. Shape the dough into a disc, and chill for 45 minutes.

Right before you are about to divide and roll out the dough, start making the filling. Add the shredded coconut and lemon zest into a bowl. Rub everything together with your fingers, making sure to break up any clumps of lemon zest.

Melt the butter in the saucepan. Pour the melted butter and milk over the coconut and lemon zest. Mix everything together and set aside to let the coconut hydrate.

Preheat the oven to 350ºF (175ºC) and position an oven rack to the center of the oven. Line 2 large half sheet baking pans with parchment paper.

Divide the dough into 18 pieces of about 27 to 28 grams each. Roll each piece into balls, about 1 inch in diameter.

Lightly flour a work surface. Take one ball and flatten it slightly with the palm of your hand. Roll out the dough until you get a circle that’s about 3 ¼ to 3 ½ inches wide. Press the rolled out dough into the tart tin, making sure that the dough is flush with the tin and it reaches all the way to the top of the tin.

After you finish rolling out the dough for 9 tins, transfer the crusts to a plate and refrigerate. Then, finish rolling out and shaping the remainder of the dough balls and chill the dough.

Add the powdered sugar, milk powder, egg, vanilla extract, lemon extract (if using), baking powder, and salt to the coconut mixture. Mix until well combined.

Arrange the first batch of 9 chilled tart tins onto a baking sheet. Scoop about 2 ½ tablespoons of filling into each tin. Bake the tarts for 15 minutes. Then, rotate the pan and bake for another 10 to 13 minutes, until the crusts have a rich golden color and the tops of the tarts have started to turn golden brown.

Remove the tarts and let them cool in the tins for 15 to 20 minutes, until they’re cool enough to handle. Invert a tart over one hand and use your other hand to carefully lift and remove the tin off the tart. Transfer the tart to a cooling rack or plate and continue unmolding the remaining tarts.

Serve the tarts slightly warm or completely cooled. Store the tarts at room temperature for 2 to 3 days or refrigerator for 5 to 6 days.