In a food processor, combine the butter and sugar. Pulse until smooth and blended.

Add the whole egg and mix until incorporated. Then add the egg yolk and vanilla, pulsing again to combine.

Sift the flour and salt together, then add to the wet mixture. Pulse until a dough forms—don’t overmix.

Wrap the dough in plastic wrap and chill in the fridge for 1 hour.

Preheat oven to 140°C (290°F). Brush 20 mini tart tins with melted butter.

Roll the chilled dough on a lightly floured surface to about 1/4 inch thickness. Cut out circles slightly larger than your tart tins and press into each tin gently.

Line each tart shell with parchment and fill with uncooked rice or pie weights. Bake for 15 minutes or until light golden brown. Remove and let cool.

In a mixing bowl, whisk together the condensed milk, egg yolks, lemon juice, and zest until smooth.

Fill the cooled tart shells nearly to the top with the lemon mixture. Bake at 160°C (320°F) for 15 minutes until the centers are just set. Let cool in tins for 15 minutes, then remove to cool completely on a wire rack.

Place the egg whites and sugar in a heatproof bowl over simmering water. Whisk constantly until the sugar has dissolved completely.

Transfer to a stand mixer and beat on medium-high speed for 10 minutes until stiff, glossy peaks form.

Pipe the meringue onto each tart and use a kitchen torch to toast the peaks until golden.