Line an 8x8 inch baking pan with parchment paper, leaving overhang on two sides for easy removal. Lightly grease the parchment.

In a medium saucepan over low heat, combine sweetened condensed milk, butter, and salt. Heat, stirring constantly, until butter is melted and mixture is smooth and hot. Do not boil.

Place semi-sweet and white chocolate chips in a large heatproof bowl. Pour the hot milk mixture over the chips. Let sit for 2-3 minutes to soften.

Add vanilla and cinnamon. Whisk the mixture steadily until the chocolate is completely melted and the fudge is smooth and glossy.

Pour the fudge into the prepared pan and smooth the top with a spatula. Tap the pan gently on the counter to remove air bubbles.

Refrigerate for at least 4 hours, or until very firm.

Use the parchment paper to lift the fudge block from the pan. Using a Christmas tree-shaped cookie cutter, cut out trees. Re-roll scraps and cut more trees.

Press one side of each fudge tree into the green sanding sugar. Decorate with assorted sprinkles for ornaments and a star for the top.

Store in an airtight container in the refrigerator. Serve slightly cool.