In a small bowl, add egg yolks, powdered stevia, salt, lemon and lime zest, whisk to combine, set aside.
In another small bowl combine hazelnut flour, buckwheat flour and flaxseed meal, stir to combine, set aside.
In a medium bowl, beat butter with an electric mixer until light and fluffy. Beat in egg mixture until combined, then add flour mixture and beat until just combined. Wrap dough and refrigerate for 1 to 2 hours.
To make filling, combine yogurt, stevia, alcohol free vanilla, lemon and lime zest in a medium bowl, stir to combine. Refrigerate filling until ready to use.
Brush 8 cups of a 12 cup muffin tin with oil, such as olive or coconut. Line the bottoms of the prepared cups with a circle of parchment paper, set pan aside.
Divide dough into eight portions. Roll portions into the shape of a ball and flatten each to ¼ inch thickness between parchment paper. Fit flattened discs into prepared cups of muffin tin, then place tin in freezer for 30 minutes.
Preheat oven to 350 degrees (177 degrees C). Bake tart shells for 15 minutes, cool 15 to 20 minutes. Remove tart shells from tin by placing a baking sheet on top of muffin tin and invert pans. Lift muffin tin, re-invert tart shells onto a rack and cool completely.
Spoon filling into tart shells, garnish with thin slices of lemon and lime, serve. Tarts can also be chilled before serving, up to 8 hours.
Garnish with lemon and lime slices.