1. Preheat the oven to 325°F. Grease a 9 by 9-inch baking pan, then sprinkle with a little gluten-free flour mix, tapping out any extra.
2. Combine the unsweetened chocolate and shortening in a microwave-safe bowl and melt in the microwave, stopping to check and stir every 30 seconds. (Alternatively, you can melt the chocolate and shortening in a double boiler.) Once melted, stir in the sugar and prune purée. Mix well, add the vanilla, and beat until smooth.
3. Whisk together the flour mix, xanthan gum, and baking powder. Add to the chocolate mixture in three batches, stirring well after each addition. Beat until smooth. Fold in the chocolate chips.
4. Spread the batter in the prepared pan, smoothing down the top with the back of a rubber spatula or large spoon. Bake in the center of the oven for 55 minutes, rotating the pan halfway through. Bake until the top looks glossy and the brownie is just beginning to pull away from the sides of the pan.
5. Let the brownies cool completely in the pan, then cut into squares. Remove from the pan and enjoy. Store in an airtight container.