For the crust, in a food processor combine flour, xanthan, nuts, sugar and salt, pulsing until pecans are finely ground. Add butter, pulsing until pea-size crumbs form. With machine running drizzle in just enough ice water until dough comes together. Wrap dough in plastic wrap and chill for 45 minutes.

You can shape the tart shells in a few different ways: roll about 1/2 Tbsp of dough into a ball, place into greased mini muffin pan and use a tart forming tool to shape the tart by moving the tool in circles. Or, roll out dough to 1/8" thick then cut out using a 2-3" circle cookie cutter. Or finally, and my favorite the 'flatten & slap' method (I just made it up :) Take about 1/2 Tbsp of dough into your hands and roll it into a ball. Squish the ball flat between your palms, then slap it back and forth until it thins out to 1/8". Gently place into greased pan, ruffling edges as needed to create a cup shape. Bake at 375°F for 20 minutes or until lightly golden brown. Allow to cool completely on wire rack.

For the pastry cream, in a saucepan over medium heat bring to a simmer the milk, ⅓ cup sugar and the vanilla. In large bowl, whisk together ½ cup buttermilk, egg yolks, cornstarch and remaining ⅓ cup of sugar. While whisking constantly, slowly drizzle in (a tablespoon at a time) the hot milk mixture. Keep adding more of the hot milk until the egg mixture warms up. At which point you can slowly pour the whole mixture back into the saucepan. The eggs must be gradually warmed up in this way, otherwise they will curdle.

Bring the entire mixture to a boil over medium heat, and boil for another 3 minutes until thick. Remove from heat and stir in the remaining ¼ cup of the buttermilk. Pour into a bowl, cover surface with plastic wrap and chill for 1 hour.

Just before serving, carefully spoon some custard into each baked tart shell, top with fresh berries and dust with some powdered sugar.