Preheat your oven to 200°F and line two baking sheets with parchment paper. Make sure your mixing bowl and beaters are completely clean—any oil residue stops egg whites from whipping properly. I learned this the hard way after a cloudy bowl ruined my first batch.
Add your 3 room-temperature egg whites and cream of tartar to the bowl. Beat on medium-high speed for about 2 minutes until soft peaks form. You'll see the mixture get frothy and thick—that's exactly what you want at this stage.
Gradually sprinkle in the granulated sugar while continuing to beat, adding it about 1 tablespoon at a time. This takes roughly 8-10 minutes total. The meringue transforms into glossy, stiff peaks that hold their shape perfectly when you lift the beaters.
Fold in the vanilla bean paste gently using a rubber spatula. Transfer to a piping bag fitted with a small round tip or star tip. Pipe small kiss-shaped portions onto your prepared baking sheets, spacing them about 1 inch apart.
Bake for exactly 2 hours at 200°F until the meringue kisses are crispy and pale. They shouldn't brown at all—if they're turning tan, your oven runs hot and you need to lower the temperature to 175°F. Mine always need that full 2 hours, and they cool completely on the baking sheet afterward.
While the meringue bakes, prepare the white chocolate buttercream filling. Melt the white chocolate chips gently in a microwave-safe bowl in 30-second bursts, stirring between each one until smooth. Don't let it get too hot or it'll separate.
Beat softened butter and powdered sugar together until fluffy, about 2 minutes. Fold in the melted white chocolate, milk, orange zest, sea salt, and lemon juice. Stir until combined and spreadable. Not gonna lie, this filling smells absolutely amazing.
Once the meringue kisses cool completely, pipe or spread a small dollop of filling onto the flat side of one kiss. Press it against another kiss to create a sandwich. Sprinkle rainbow sprinkles over the tops immediately while the filling's still slightly soft so they stick.
