Preheat the oven to 180°C fan (200°C conventional). Line a rectangular baking tray with parchment paper and unroll the puff pastry onto it.
Using a sharp knife, lightly score a 1-2 cm border around the edges of the pastry without cutting through. Prick the centre all over with a fork to stop it puffing up too much during baking.
Peel the apples and slice the flesh away from the cores in four sections. Lay each ‘cheek’ flat and slice thinly. Keep the slices slightly overlapped and grouped together for easy arrangement on the tart.
Sprinkle the sliced apples with a little brown sugar to enhance sweetness, adjusting based on how tart your apples are.
In a bowl, mash together the softened butter, 75g brown sugar, vanilla extract, cinnamon and a pinch of salt until smooth and spreadable.
Spread the cinnamon butter mixture over the centre of the pastry, avoiding the border. Arrange the apple slices on top, either in mounds or rows. Brush the pastry edges with beaten egg.
Place in the oven and bake for 20-25 minutes, or until the pastry is golden and crisp and the apples are soft and slightly caramelised.
Warm the apricot jam in the microwave for 15 seconds until loosened. Brush it gently over the warm tart for a glossy finish. Slice and serve warm or at room temperature with a scoop of vanilla ice cream.