Preheat the oven to 350°F. degrees.

To make the pastry, combine ½ cup sugar, the softened butter, and the egg yolks in a food processor bowl, creaming them together with a fork before inserting the stainless-steel blade. With the blade in place, gradually pulse in the flour just until a crumbly dough forms; do not overmix. Pat the pastry into a greased 9-by-13-inch glass baking dish, forming a thin uniform layer about ½ inch thick. Bake in the oven on a middle rack until golden, 15 to 20 minutes. Put the jam in a microwave-proof bowl and microwave it for 15 to 20 seconds. Then, spread it evenly on top of the hot pastry. Set aside.

Make the meringue topping by combining the egg whites and cream of tartar in a clean mixing bowl. While beating with an electric mixer at medium-high speed, gradually pour in the remaining 1 cup sugar until the egg whites are stiff enough to hold firm peaks that don’t droop when the beater is lifted out. Spread the meringue over the jam. Bake until golden, about 25 minutes.

Let the Vienna Squares cool for a few minutes before cutting  into squares and serving.