Line 12 to 14 muffin cups with paper liners. Preheat oven to 350° F.
Sift the flour, arrowroot starch, baking powder and salt into a large bowl.
In another bowl, whisk the oil, honey, egg yolks, lemon zest, lemon juice, water, and vanilla.
Add to the flour mixture and whisk until smooth.
In a large bowl of a heavy duty mixer fitted with the whisk attachment, whip the whites on medium speed until foamy. Add the cream of tartar and continue whipping until the whites almost form firm peaks. To prevent overwhipping, remove from the mixer and finish whipping by hand. Do not whip until the egg whites are dry.
Stir about 1/3 of the whites into the batter to lighten it. Gently fold in the remaining whites. Divide into lined muffin tins.
Bake until the cupcakes are light golden and the tops spring back when gently pressed with a finger, 15-20 minutes. Cool.
Cut a small hole out of the top of each cupcake. (see note) Fill the hole with Honey Lemon Curd.
Make Honey Lemon Curd first. Refrigerate or freeze the unused egg whites until you need them for these cupcakes. If the whites are frozen, let thaw. (2 tablespoons of egg whites roughly equals 1 egg white.)
Arrowroot lightens the whole wheat flour.
If you wish, sprinkle confectioners’ (powdered) sugar lightly over the cupcakes using a fine sieve.