Heat oven to 350 degrees F. Line an 8x8 square pan with foil, extending the sides of the foil over the edges of the pan. Grease foil with cooking spray; set aside.
Whisk water, eggs, and Meyer Lemon Filling Mix until blended. Add in the lemon zest and mix well. Set aside.
Stir melted butter into crust mix until crumbly dough forms. Press the dough firmly and evenly into the bottom of the pan. Bake the crust for 12-14 minutes.
Pour the lemon filling evenly over the crust and return to the oven to bake 20-22 minutes.
While lemon bars bake, make your meringue. In a clean bowl, whip egg whites, cream of tartar, and vanilla on HIGH speed until stiff peaks form, slowly adding in the sugar a little at a time. Meringue should be stiff and glossy. Spread over the still-hot lemon bars evenly, making small swoops and swirls with the meringue.
Return the bars to the oven and bake for 7-10 minutes longer, watching the meringue carefully to prevent it from browning or burning too quickly. Meringue should be lightly toasted on top. Remove from the oven and let cool completely, then refrigerate for at least 2-3 hours to firm and set before cutting into squares.