In a large bowl, whisk together 1½ cups (188 grams) flour, granulated sugar, and 1 teaspoon (3 grams) salt. Stir in 7 tablespoons (77 grams) oil just until dry ingredients are moistened.
In a small bowl, whisk together egg yolk and cold milk. Add egg yolk mixture to flour mixture, 1 tablespoon (12 grams) at a time, stirring until a dough forms.
Lightly spray a 12-cup muffin pan with cooking spray. (See Note.)
Divide dough into 12 portions (about 24 grams each); place 1 portion in each prepared muffin cup, pressing into bottom and ¾ inch up sides. (Crusts will be about ⅛ inch thick.)
In a medium bowl, whisk together brown sugar, orange zest (if using), remaining 2 teaspoons (6 grams) flour, and remaining ⅛ teaspoon salt. Whisk in honey, egg, vanilla, and remaining 1 tablespoon (11 grams) oil until well combined. Stir in toasted walnuts. Divide sugar mixture evenly among prepared crusts (about 4 teaspoons [24 grams] each), stirring filling every few scoops to evenly distribute walnuts. Top each with 1 walnut half.
Bake until filling is puffed and set and crust is golden brown, 15 to 17 minutes. Let cool in pan for 15 minutes. Carefully remove from pan, and let cool completely on a wire rack. Garnish with confectioners’ sugar and melted chocolate, if desired.
Notes
Line each greased muffin cup with a 7x1-inch parchment strip to help get baked tassies out of the pan more easily