Preheat the oven to 325ºF and grease a 24-cup metal mini muffin pan. You can also use silicone, although the shells may take a bit longer to bake.

In a large bowl, whisk together the almond flour, powdered sweetener, baking powder, and salt. Stir in the egg and melted butter until the dough comes together. Gather the dough into a ball.

Dust a work surface lightly with more almond flour and transfer the dough to the work surface. Cover the dough with waxed paper or parchment paper and roll out to an even ¼" thickness.

Cut out the tart crusts using a 2” round cookie cutter. Lift the cut-outs carefully with an offset spatula and press each lightly into their own cup in the prepared mini muffin pan. Don’t press too firmly into the bottom of the cup, as they may stick after baking (see notes).

Gather the scraps and re-roll dough as many times as needed to make 24 tart crusts in total (see notes).

Bake the crusts 10 minutes, then remove and let cool completely to firm up.

Gently press a piece of brie into each tart shell. Add a teaspoon of sugar-free cranberry sauce on top of each bite.

Return to the oven and bake another 12 to 15 minutes, until the brie is bubbly and the edges are just golden brown.

Remove and let cool in the pan 10 to 15 minutes, then gently loosen and lift bites out with a small sharp knife. Sprinkle with chopped walnuts and rosemary leaves if desired.