Measure cold butter, cut it into small pieces and place it in the freezer for 10 minutes.
Pre-heat the oven at 180 degree C.
In a bowl, mix together the oats flour, all-purpose flour, sugar and baking powder.
Next add the cold butter cubes and using your finger tips rub it into the flour till you get a breadcrumb-like consistency.
Start adding cold water - one tsp at a time - and bringing the flour together into a soft pillowy dough. Do not add too much water.
Place the dough in the fridge for 5-10 minutes and then transfer to your tart tin. Use your fingers to line the sides first and then the base. Use a knife to trim any excess dough over the sides of the tin.
Dock/prick the dough at the base and bake in a pre-heated oven at 180 degree C for 25-30 minutes or till the sides start to turn golden-brown.
Remove the tin and let it cool for 10-15 minutes. Meanwhile prepare the ganache.