Berry and Custard Toaster Tarts

STEP 1: MIX THE PASTRY

Whisk the gluten-free flour blend, xanthan gum (if using), sugar and salt in a large bowl. Toss in the butter cubes and press each one flat between your fingertips, leaving large flakes. Mix the egg, cider vinegar and most of the ice water, add to the bowl and bring together into a shaggy dough, adding the remaining water only if needed. Shape into a rectangle, wrap and chill for 30 minutes.

STEP 2: MAKE THE COMPOTE

While the dough chills, simmer the berries, sugar and lemon juice in a small saucepan for 6 to 8 minutes, crushing some of the berries, until jammy. Mix the cornstarch with the cold water, stir it in and boil for 1 minute until thick. Cool completely, then chill. It must be cold and thick before it goes into the pastry, or it will boil out.

STEP 3: MAKE THE CRÈME PÂTISSIÈRE

Heat the milk in a medium saucepan until just simmering. Whisk the egg yolks, sugar and cornstarch in a bowl until pale, then gradually whisk in the hot milk. Return to the pan and whisk over medium heat until it boils and thickens, then cook for 1 minute more. Off the heat, whisk in the vanilla and butter. Pass through a fine-mesh sieve into a dish, press plastic wrap onto the surface and chill until firm, at least 1 hour.

STEP 4: LAMINATE WITH SIX FOLDS

Between two sheets of parchment paper, roll the dough into a rectangle about 6 x 15 in. Peel off the top sheet and use the bottom sheet to lift and fold the dough in thirds like a letter. Turn it 90° and repeat. Wrap and chill for 20 minutes. Repeat twice more for six folds in total. If the dough cracks, patch it, chill it and carry on. Chill for 30 minutes after the final fold.

STEP 5: CUT THE RECTANGLES

Roll the dough between parchment to 1/8 in thick and trim to a neat 12 x 16 in rectangle. Cut sixteen 3 x 4 in rectangles with a pastry wheel or sharp knife. Transfer to two parchment-lined baking sheets and chill for 10 minutes if they have softened.

STEP 6: FILL THE TARTS

Beat the egg with the milk to make an egg wash. Whisk the chilled crème pâtissière until smooth. On eight of the rectangles, spread 1 tbsp crème pâtissière, leaving a 1/2-in border, and top with 2 tsp compote. Do not overfill. Brush the borders with egg wash.

STEP 7: SEAL AND VENT

Lay the remaining rectangles on top, press the edges firmly to seal and crimp with a fork. Pierce each top four or five times with a skewer so the filling cannot burst through. Chill for 15 minutes while the oven heats to 400°F (200°C).

STEP 8: BAKE

Brush the tops with egg wash using a pastry brush. Bake for 22 to 26 minutes, rotating the sheets halfway, until deep golden, crisp at the edges and firm underneath. Gluten-free pastry colors more slowly than wheat pastry, so judge by color and a firm base rather than time. Cool completely on a wire rack.

STEP 9: ICE AND DECORATE

Mix the powdered sugar with enough milk or strained compote juice to make a thick icing that holds its shape. Spread over the cooled tarts, leaving the crimped border showing, and scatter with sprinkles at once so they stick. Best eaten the day they are made; because of the custard, store chilled and eat within 2 days.