For filling, cover prunes with water in a saucepot and simmer uncovered for 10 minutes. Drain liquid, reserving 1/2 cup. Puree prunes in a food processor. Spoon puree back into pot, add reserved liquid and sugar, vanilla and cinnamon. Cook over low heat, stirring regularly, until filling becomes thick.
For dough, combine flour and salt. In a separate bowl, whisk eggs, milk and water. Add liquid to flour and stir until blended. The dough should be soft enough that it sticks to the bottom of the bowl, but not so soft that it doesn’t hold its shape. Cover dough and let rest at room temperature for 1 hour, or chill dough until ready to roll, pulling out of fridge an hour before using.
On a lightly floured surface, roll out dough as thinly as possible without tearing. Use a round cutter 3 inches (8 cm) in diameter and cut out circles of dough. Place 1 Tbsp (15 mL) of potato filling into the center of each perogy and pinch the perogy edges closed. Perogies can be frozen on a baking sheet or kept chilled until ready to cook.
To serve, melt butter with brown sugar in a sauté pan over medium-high heat. Once mixture is bubbling, stir in lemon juice and vanilla. Reduce heat to medium-low. In a pot of boiling salted water, cook about half of the perogies until they float, about 5 minutes. Remove from water with a slotted spoon, add to toffee sauce and cook remaining perogies. Stir to coat, then serve with sour cream.