Add the egg whites, sugar, corn syrup, cream of tartar, and salt in the clean bowl of an stand mixer and nestle it in a saucepan of simmering water over medium-high heat. Do not allow the bottom of the bowl to touch the water. Whisk until the mixture is opaque, frothy, the sugar is melted (rub some between two fingers, if it feels gritty, keep whisking) and warm to the touch. Remove the bowl from the heat and transfer it to the stand mixer fitted with the whisk attachment. Mix on medium-high speed until stiff and glossy and cool to the touch, about 3-5 minutes. Add in the vanilla and run the machine for 30 seconds more or until combined. For the round tarts I used an extra large ice cream scoop to mound the marshmallow on there. For the rectangle tart I found it easier to throw it in a piping bag and pipe it onto the tart. The pudding is soft, so take care to gently smear the marshmallow over the top. Use a torch to char the meringue using a circular motion, focusing on the peaks.