Fizzy QR Code Tart

STEP 1: MAKE AND CHILL THE PASTRY

Pulse the all-purpose flour, powdered sugar, almond flour and salt in a food processor. Add the cold butter and pulse until the mixture looks like fine breadcrumbs. Add the egg yolk and pulse until the dough just comes together, adding the ice water only if needed. Flatten into a disk, wrap and chill for 1 hour.

STEP 2: BLIND BAKE THE SHELL

Roll the pastry to 1/8 inch (3 mm) thick and line the tart pan, leaving a little overhang. Chill for 20 minutes. Heat the oven to 350°F (175°C). Set the pan on a baking sheet, line with parchment and pie weights, and bake for 18 minutes. Remove the weights and bake 8-10 minutes more until golden. Brush the inside with the beaten egg white and bake 2 minutes to seal. While the shell is still warm, trim the overhang flush with a sharp knife for a clean, straight rim. Lower the oven to 300°F (150°C).

STEP 3: MAKE THE STRAWBERRY-LEMON CUSTARD

Blend the strawberries until smooth and press through a fine-mesh sieve; you should have about 1 cup (240 ml). Simmer the purée in a small pan for 5-7 minutes until reduced to about 2/3 cup (160 ml), then cool completely. Whisk the granulated sugar, eggs and egg yolks until smooth, then whisk in the heavy cream, lemon zest, lemon juice and cooled purée. Strain into a pitcher and skim off any foam.

STEP 4: BAKE THE CUSTARD

With the shell still on its baking sheet, set it in the oven and pour in the custard to within 1/8 inch (3 mm) of the rim. Bake at 300°F (150°C) for 30-40 minutes, until the edges are set and the center has a slight, even wobble. Cool on a rack, then chill for at least 3 hours. The surface must be firm, cold and dry before anything goes on top.

STEP 5: BAKE THE COOKIE TILES

Beat the softened butter and granulated sugar until pale, then beat in the egg yolk and vanilla extract and mix in the flour. Roll between two sheets of parchment to 1/8 inch (3 mm) thick and freeze for 15 minutes. Heat the oven to 350°F (175°C). Using a ruler and sharp knife, cut 1/2-inch (1.3 cm) squares from the frozen dough so every edge stays sharp; you need about 36, plus a few spares. Bake on a parchment-lined baking sheet for 7-9 minutes, until just pale gold at the edges. Trim any spread with the knife while the tiles are still warm, then cool completely.

STEP 6: MAKE THE FIZZY WHITE CHOCOLATE-LIME LAYER

Spread the lime zest on a paper towel and leave it to dry for 10 minutes. Mix the powdered sugar, citric acid and baking soda into a sherbet powder. Set aside one-third of the white chocolate. Melt the remaining two-thirds in a dry bowl over barely simmering water, stirring, until it reaches 110°F (43°C). Remove from the heat, wipe the base of the bowl dry, and stir in the reserved chocolate until it melts and the mixture cools to 82-84°F (28-29°C). Stir in the dried lime zest, the sherbet powder and the popping candy, if using. Work quickly and keep every trace of water away, as moisture sets off the fizz.

STEP 7: SHAPE THE FIZZY LAYER AND DIP THE TILES

Draw a 5 5/8-inch (14.3 cm) square on parchment and flip it over. Spread all the fizzy chocolate inside the lines in an even layer about 1/4 inch (6 mm) thick with the offset spatula. When it is almost set, trim the edges straight with a warm, dry knife, then chill for 10 minutes until firm. Melt the dark chocolate gently over barely simmering water. Dip the cookie tiles one at a time, let the excess drip off, and set them on parchment to firm up. Keep about 1 tablespoon of the dark chocolate warm in a piping bag.

STEP 8: BUILD THE QR CODE

Lift the set fizzy chocolate square onto the center of the chilled tart. Using the printed 9 x 9 grid template as your guide, place the dark tiles one row at a time with tweezers: a solid 3 x 3 block in three corners to form the finder squares, then a random-looking scatter of single tiles between them, leaving the white chocolate showing through the empty cells. Keep every gap an even 1/8 inch (3 mm). Leave the center cell empty and pipe a small tent outline there with the reserved dark chocolate.

STEP 9: FRAME AND SERVE

Warm the jam with the lemon juice, then brush a glossy ring of it over the exposed custard between the chocolate square and the pastry rim. Pat the cut sides of the halved strawberries dry and arrange them around the edge, cut side up. Chill for 15 minutes, then slice with a hot, dry knife, wiping it clean between cuts, and serve cold. Store uncovered in the fridge; the fizz is strongest on the day the tart is made.