Preheat oven to 350ºF. Grease or paper-line 12 standard-sized muffin tin cups.
Whisk flour, baking powder, baking soda, and salt in a medium bowl to blend. Combine buttermilk, lemon juice, and zest in a small bowl.
In the bowl of a standing electric mixer, beat butter until smooth. Slowly add sugar and beat until well-blended. Beat in eggs, one at a time, scraping down the sides to ensure even mixing. Add vanilla extract and mix well. Stir in dry ingredients until just combined. Add wet ingredients, stirring to incorporate evenly while taking care not to over mix.
Scoop the thick batter into the prepared muffin cups, filling each one about halfway. Bake until the tops look firm and a skewer inserted in the center of a cupcake comes out mostly clean, about 20 minutes.
While the cupcakes bake, prepare the lemon syrup. Combine lemon juice and sugar in a small saucepan over low heat. Stir until the sugar dissolves. Set aside.
As soon as the cupcakes come out of the oven, brush the lemon syrup over the tops with a pastry brush, avoiding the area close to the edges. Repeat until all the glaze is used. Let cupcakes cool for 5 to 10 minutes in the pan before removing them to a cooling rack.
To make the meringue, combine egg whites and cream of tartar in the bowl of a standing electric mixer. Set aside.
Combine water and sugar in a medium saucepan over high heat. Bring to a full boil, reduce heat to medium, and continue cooking for 4 minutes.
In the meantime, beat eggs with the mixer's whisk attachment at medium speed until soft peaks form (lift the mixer and look for gentle peaks that slowly collapse), about 2 minutes.
With the mixer running – working very carefully – add the hot sugar to the egg whites in a slow, steady stream. Increase speed to high and beat for about 4 minutes or until the mixture is just warm to the touch. Spoon or pipe meringue atop cupcakes and lightly brown with a blowtorch or under the broiler.