CrustIn the bowl of an electric mixer fitted with the paddle attachment, cream the butter and 3/4 cup of the sugar on medium speed for about 8 minutes, until light and fluffy. Add the orange extract, orange zest, and egg, and mix until incorporated. Add the flour, milk power, baking soda and baking powder and mix for 8 more minutes on medium speed, or until the dough is smooth. Chill in one piece.TartOn a lightly floured surface roll the dough out to 1/4 inch thick. If using individual tart pans, use one pan as a guide, and cut circular pieces of dough about 1/2" outside the edge of the pan. Press the crusts into the tart pans so that the crusts cover the bottom and the ridged sides of the pans. Trim the dough so that it does not overlap the lip of the pan (as you might do with a pie crust). If using one large pan, follow the same instructions for forming the crust, and use the rolling pin to lift the dough into the pan. Refrigerate for at least 30 minutes.
Preheat the oven to 350F
Prick the crust several times with a fork or toothpick to prevent it from bubbling up, and bake until dark golden brown, about 15 minutes for the tartlets, and 20 minutes for the large tart. For the large tart, you can place a piece of parchment paper or coffee filter in the center of the crust, and weight it down with rice or dried beans before baking. (I prefer to bake my crust longer than many chefs, to a dark golden brown, so they remain crisp after the addition of the filling.) Let cool slightly. Unbaked tart shells will keep for up to 2 months in the freezer.
In a medium bowl, stir together the passion fruit puree, sugar, sweetened condensed milk, and eggs, and transfer the mixture to the top of a double boiler over gently simmering water. Heat, whisking occasionally to prevent lumps from forming, until the mixture reaches a thick custard-like consistency, 35 to 40 minutes. Remove from the heat and let cool for about 10 minutes. Stir in the butter until melted. (The filling can be made ahead and refrigerated for up to 3 days.)
Unmold the tart shells and fill each shell with passion fruit custard, and refrigerate until set, about 2 hours. Filled tarts will keep for up to 2 days in the refrigerator.