Preheat the oven to 350 degrees F. Grease a 9x9 inch baking pan with butter. Then, line with a strip of parchment paper cut to fit, leaving several inches of overhang on either side.
In a large bowl, melt the butter in the microwave in 30-second bursts, stirring between each one. If the butter is hot, allow it to cool a bit before moving on.
To the melted butter, add the flour, sugar, salt, and orange zest. Mix until well combined.
In a small bowl, mix together the plum jam and orange juice
Using your hands, press half of the dough into the bottom of the prepared baking dish. Then, spread out the jam mixture. Crumble the other half of the dough evenly over the jam.
Bake for 35 to 40 minutes, just until the top begins to very lightly brown.
Allow the bars to completely cool in the pan, about 2 hours. Then, remove them from the pan using the parchment paper handles.
Using a fine-mesh sifter, dust the top of the bars with powdered sugar.
Cut the bars in half, then cut each half in half again to give you 4 rows. Then, rotate the bars and repeat the other way. This will yield 16 small bars.