In a food processor, pulse together the flour, sugar, and salt. Drop in the cold butter and pulse until the mixture resembles a crumble. Add the egg yolk and half the milk, and pulse. Add more milk as needed until the dough comes together in the food processor.

Divide the dough in 2, patting and pressing each half to combine and form rectangles. Wrap each in plastic wrap and chill for at least 45 minutes.

Working with half the dough, roll it out very thin into to a large rectangle and trim into a 9"x16" (if you can, if not, don't worry). Cut into 12 pieces that are ~3"x4" (or if your dough is too uneven, cut less pieces and reroll the scraps!). Brush 6 of the pieces with the whisked egg and dollop 1 tablespoon of rhubarb apple butter along the center of each, leaving a border. Top each with another rectangle of pastry and press the edges to really seal them. Transfer to a parchment lined tray and refrigerate for about 20 minutes.

Preheat the oven to 375ºF. Transfer the chilled pop tarts to a parchment-lined baking sheet and dock the tops of each to make lots of little air holes for steam to escape. Bake for about 20 or so minutes until the edges are golden brown. Transfer to a rack to cool completely.

Repeat to form 6 other pop tarts with the other half of the dough.

Make the glaze by whisking the hot milk into the icing sugar with the corn syrup and the vanilla. The glaze will be thick but fluid. You can add a couple drops of lemon juice to help perk up the flavour.

Spread a thin layer of glaze on top of each pop tart and top with a generous amount of sprinkles.