Preheat oven to 350 degrees F. Prepare a muffin pan with cupcake lines or baking spray.

In a large bowl, whisk together flour, sugar, and baking powder. Using a hand mixer on medium speed, add butter, egg whites, vanilla, sour cream, milk, and Bailey's just until smooth.

Fill cupcake liners until about 2/3 full. Bake 18-20 minutes, until a toothpick inserted into the center comes out clean.

Cool completely before icing.

In a double boiler, over simmering water, whisk the egg white and sugar until the mixture is warm, thin, foamy, and syrupy. Make sure the bottom of the bowl doesn't touch the water or you risk scrambling the egg whites.

Transfer the mixture to a large bowl and add the salt and cream of tartar. With a hand mixer or stand mixer, beat a medium-high speed until glossy, stiff, but not dry, about 10 minutes.

By now, the egg whites should be cooled to room temperature. If the meringue still feels warm, reduce speed to low and continue to mix until completely cool.

Gradually at the butter and continue to beat on medium-low speed. The meringue will start to deflate and will initially appear lumpy -- keep beating. One the meringue appears smooth, add the vanilla and beat thoroughly. Add the ground pistachios and beat thoroughly.

The meringue is easiest to work with at room temperature. If piping, make sure the tip you use is large enough for any odd-size pieces of pistachio to fit through. Pipe or spread onto completely cooled cupcakes.