To make the breadcrumbs, break stale bread slices into pieces and process in a food processor fitted with a steel blade until it forms crumbs. Making sugar syrup for the filling takes time and patience. Use a sturdy heavy-bottom saucepan and thoroughly dissolve the sugar before increasing the heat. Keep a close watch on it after that as the syrup can quickly turn dark and bitter and will eventually burn. This syrup is a homemade version of the British product golden syrup, a traditional ingredient in Harry’s favorite tart. If you can get hold of it, you can use 1 1/2 cups (475 grams) golden syrup instead, and skip making your own. We’ve stenciled our tart with the Hufflepuff badger, but you could just as easily draw and use the Gryffindor lion, Ravenclaw raven, or Slytherin snake. Treacle tarts can be served hot as well as cold.