Line an 8-inch square pan with parchment paper, leaving enough excess over the sides to lift the chilled fudge out. Use a 9-inch square pan instead if you prefer thinner fudge or want more, slightly smaller trees.

Place the white chocolate chips and sweetened condensed milk in a heavy saucepan. Warm over low heat, stirring constantly, until the chips are almost completely melted and the mixture looks smooth; do not let it boil.

Remove the saucepan from the heat and stir in the vanilla extract. Add green gel food coloring gradually, stirring after each addition, until the fudge reaches the shade you want.

Spread the warm fudge evenly into the prepared pan. Immediately scatter the sprinkles, nonpareils, colored sugar, and optional small candies over the surface while it is still soft.

Refrigerate the pan for about 2 hours, or until the fudge is fully firm. Do not rush this step, since completely chilled fudge is much easier to cut cleanly.

Lift the fudge from the pan using the parchment paper and place it on a cutting board. Cut triangles with a sharp knife or use a small tree-shaped cutter, then press a pretzel-stick piece into the bottom of each tree for a trunk if desired.