Instructions:
- Preheat oven to 350°F (175°C). Line 18 cupcake cups with liners.
- Whisk flour, baking powder, and salt in a medium bowl. Set aside.
- Beat softened butter and 1 cup sugar until light and fluffy. Beat in the 3 egg yolks one at a time until incorporated, then mix in 1 teaspoon vanilla.
- Gradually add the dry mixture, alternating with the ¼ cup of whole milk, mixing only until just combined. Do not overmix.
- In a separate, clean bowl, beat the 3 egg whites with the cream of tartar until soft peaks form. Gradually add the ¾ cup sugar and beat until stiff, glossy peaks form.
- Gently fold one-third of the stiff egg whites into the main batter to lighten it, then gently fold in the remaining whites until no streaks remain.
- Divide batter evenly among 18 liners (filling about 2/3 full). Bake for 18–22 minutes, or until a toothpick inserted comes out clean. Let cool in the tins for 5 minutes.
- While cakes bake, whisk together the evaporated milk, condensed milk, heavy cream, and soak vanilla in a bowl.
- While the cupcakes are still warm, use a skewer or fork to poke 5–7 deep holes into the top of each cupcake. Place the cupcakes into a shallow baking dish and slowly and evenly drizzle the milk mixture over all 18 cupcakes. Allow them to sit and absorb the liquid completely (about 20–30 minutes).
- Prepare the meringue frosting: Gently heat the 4 remaining egg whites and the sugar (for frosting) in a heatproof bowl over simmering water (double boiler) while whisking constantly until the sugar dissolves and the mixture reaches 160°F (71°C). Transfer to a mixer, add cream of tartar, and whip until stiff, cool, glossy peaks form.
- Pipe or spread the meringue frosting generously on top of the soaked cupcakes and chill for at least 2 hours before serving.