Preheat oven to 170 degrees Celsius (340 degrees Fahrenheit).
Place the coconut oil in a large bowl and melt over a saucepan of boiling water or in the microwave.
Add the maple syrup, milk, ground almonds, brown rice flour and salt and mix well - add an extra splash of milk if it’s looking too dry to combine.
Transfer the mixture into a square or rectangular baking tin lined with greased baking paper – I used a 23cm/9inch square baking tin.
Use your fingers to push the mixture all the way to the edges, then use a spatula to press down on it until the surface is completely even.
Bake in oven for 20-25 minutes until golden brown and firm to the touch.
Leave to cool as you make the custard, although it doesn't matter if it hasn’t cooled completely before pouring on the custard.
For the lemon custard:
Measure out the milk in a jug or measuring container.
Pour the milk into a pan, leaving behind a few tablespoons of milk in the jug.
Heat up the milk in the pan.
Add the custard powder to the remaining milk in the jug and mix well, ensuring it has dissolved completely.
Once the milk in the pan is steaming, add approximately half of the hot milk to the bowl with the dissolved custard powder, mix well and transfer the custard powder mixture to the pan.
Mix well and keep stirring on a low heat for around 5 minutes, until thickened.
Add the lemon juice and maple syrup.
Taste and add extra lemon juice or maple syrup if desired.
Pour the lemon custard over the shortbread base until it’s completely evenly covered and make sure the surface is smooth.
Tap the dish gently on the counter a few times to get rid of the air bubbles.
Leave to cool then keep in the fridge for at least 4 hours until it firms up (may take slightly longer depending on the type of custard powder you use, the temperature of your fridge etc).
Cut into squares once it’s completely firmed up.
Keeps covered in the fridge for up to a few days.
*You can alternatively use almond flour.**You MUST use brown rice flour - white rice flour won't create a crunchy shortbread-like texture.***You can use any type of plant-based milk, though results do vary depending on which you choose: cashew, oat or coconut milk make a super thick and creamy custard, whilst almond milk will be less creamy. I like to use rice milk as it creates a just-set lemon curd-like consistency which I love.****If you can't get hold of a shop-bought custard powder, you can substitute it with the same amount cornflour (cornstarch) or arrowroot, which will result in the same texture, though the colour will be considerably less yellow. To add yellow colour, I would suggest adding a tiny pinch of saffron, which I much prefer to turmeric as it has a much milder flavour.