In a medium saucepan, melt the butter over low heat; set aside to cool. In a medium bowl, whisk together the cassava flour, arrowroot starch, baking soda, and salt; set aside.
Whisk the yogurt and egg into the melted butter. Add in the flour mixture and mix until a soft and pliable dough forms. Divide the dough into 16 pieces and press each piece into 2½ -inch fluted tart molds, starting from the bottom and working your way up the sides, until the dough is evenly distributed and about ¼-inch thick. Refrigerate the tart bases for 30 minutes.
Preheat the oven to 400°F.
To make the filling, put the yogurt, lemon juice, egg yolks, salmon, capers, and 2 teaspoons of chopped dill in a small bowl; mix well and season with salt and pepper to taste.
Bake the tart bases for 15 minutes or until the crust edges are golden. Divide the smoked salmon mixture evenly among the tart bases and return to the oven for 5 more minutes.
Let cool completely before inverting. Garnish with dill sprigs.