Make crust: combine bread mix dry ingredients and salt in a stand mixer. Dice cold butter into cubes and sprinkle over mix. Mix with paddle on high until butter is broken down to pea-sized pieces. With mixer running, add ice water a little at a time just until dough comes together. Form into 2 equal balls, wrap in plastic, refrigerate at least 1 hour (freeze one for later).

Preheat oven to 375°F. Roll one crust ball to fit pie pan. Transfer carefully and press in without stretching. Poke with a fork. Blind bake with parchment and dried beans for 20 minutes. Remove beans and bake 5 more minutes until golden. Cool.

Make curd: in a double boiler, whisk yolks, whole egg, and erythritol. Add lemon juice, zest, and salt. Whisk over simmering water until thick, doubled in volume, and holds whisk lines, 5 to 10 minutes. Strain through a fine-mesh strainer. Cool in an ice bath until just warm. Whisk in butter and vanilla. Pour into baked crust.

Make meringue: whisk egg whites and cream of tartar until frothy. Turn to high speed and add erythritol 1/2 tbsp at a time. Add vanilla if desired. Beat until stiff and glossy. Spread over lemon curd.

Optional: use a kitchen torch to lightly brown the meringue peaks. Refrigerate until set before slicing.