Well, as far as this being Harry Potter's favorite treacle tart of all treacle tarts, that might be an overstatement. After all, it's difficult--if not downright impossible--to match the mastery of the Hogwarts house-elves or the magical air of the Great Hall. Fans will know, however, that treacle tart is a favorite dessert of Harry's. Unfortunately, few in the US know the deliciousness of this British treat. Luckily, it's not difficult to make.
Step 1: Tart Pastry
2 cups all-purpose flour
1/4 cup granulated white sugar
1 tsp. salt
1 cup cold butter, cut into small pieces (leave in refrigerator until needed)
2 egg yolks
1/4 - 1/2 cup heavy whipping cream
1 tsp. vanilla extract
2 Tbsp. lemon juice
Whisk together the egg yolks, 1/4 cup heavy whipping cream, vanilla extract and lemon juice in a small mixing bowl. In a medium mixing bowl, add together the flour, sugar and salt. Put both bowls in refrigerator for about 30 minutes.
Put the dry ingredients in the food processor and pulse to combine (or if a food processor is not an option, mix it by hand). Add the butter and pulse (or cut in with a pastry blender) until texture resembles course crumbs. Put the mixture back into the medium mixing bowl and pour the small mixing bowl with the liquid ingredients over it. Stir just until combined. If dough doesn't pinch together add up to another 1/4 cup of heavy whipping cream a tablespoon at a time until it does pinch together.
Turn the dough out onto your clean work surface and divide in two. Shape each half into a flattened disc and plastic wrap it. Refrigerate for at least 2 hours (use with in 3 days).
Place one disc onto a well-floured work surface (tart dough is very sticky). Flour the top of the disc generously and the rolling pin. Roll the dough out so you have a 12" diameter. Roll from the center out, and rotate the dough occasionally to avoid sticking. Use a pastry brush to dust off the excess flour, and gently fold the dough in half. Dust the side facing up as well and gently fold in half again. Dust the showing quarter, turn over and likewise dust. Transfer the dough to a 9" tart pan and unfold. Gently push the pastry dough into the corners without stretching. Any tearing that occurs, use your fingers to patch back together. Roll the rolling pin over the pan edges to remove the excess dough. Refrigerate for about 30 minutes.
Roll the second disc out similarly. Cut 1/2" strips (pizza cutter works nicely) and place on a parchment lined cookie sheet. Refrigerate for about 30 minutes.