For the pastry: Combine flour, ground almonds, mint leaves, sugar, baking powder, and butter in a food processor. Pulse until mixture resembles rough breadcrumbs.
Add the water, 1 tbsp at a time, and pulse between additions until a rough dough comes together; add more water as needed to bring it together if too dry.
Turn out the dough onto a floured surface and knead briefly. Wrap in cling wrap chill for 1 hour.
After chilling, remove the dough from fridge and preheat the oven to 180°C (160° fan).
Roll out the dough on a lightly floured surface to about 1/2 cm | 1/4″ thickness. Cut out six rounds and use them to line the base and sides of individual tartlet tins, pressing well into the base and sides.
Cut away any overhanging excess and prick the base with a fork. Line each with greaseproof paper and fill with baking beans, arranging them on a large baking tray.
Blind-bake for about 15 minutes until golden at the edges. Remove from the oven and discard the paper and baking beans. Return the pastry to the oven to brown the bases, about 3-5 minutes.
Remove from the oven to a wire rack to cool, brushing with the beaten egg wash to seal. Cover with a tea towel as they cool. Keep the oven on.
For the filling: Combine both milks with half the sugar, peppermint extract, and the salt in a saucepan set over a medium heat.
Bring to a simmer, stirring. Whisk in matcha green tea powder until thoroughly combined. Set aside to cool.
Beat the egg yolks with the remaining sugar and cornflour in a heatproof bowl until pale and thick. Whisk in the milk mixture in a slow, steady stream until incorporated.
Pour mixture into a clean saucepan and cook over a medium heat, stirring, until simmering and thickened, about 2-3 minutes.
Strain the mixture through a fine sieve and beat in the butter by the cube until glossy and smooth. Divide between the tartlets, returning them to the oven until the tops of the filling is just set, about 10-15 minutes.
To serve: Remove from the oven and let the tartlets cool briefly. When ready to serve, dust with green tea powder and top each with a pecan half.