Heat milk over the stove until barely simmering, remove from heat.
Whilst milk is heating, mix together eggs, sugar, both flours and salt in a bowl, whisk together until combined.
Slowly pour half the hot milk into the egg mixture while whisking to temper the eggs.
Pour mixture back into the saucepan on the stove and cook over medium heat while stirring constantly.
Once thickened completely, whisk in the butter, then pour into the prepared tart shell.
Work quickly to smooth custard into tart shell, as a skin will form very quickly as it cools.
Allow to cool for half an hour at room temperature before dusting with cinnamon and transferring to the fridge to set (minimum 3 hours).
