Mirror Cake Glaze

Stir gelatin and 1/2 cup lukewarm water in a small bowl and let sit 5 minutes.
Cook sugar, condensed milk, vanilla, salt, and 3/4 cup water in a medium saucepan over medium heat, stirring occasionally, until mixture begins to bubble, about 4 minutes. Add bloomed gelatin and stir until dissolved.
Remove from heat and add chocolate chips, stirring constantly, until melted. Strain through a fine-mesh sieve into a large bowl, then divide glaze among 3 or 4 bowls. Color each with food coloring as desired. Let cool, stirring occasionally to stop a skin from forming, until glaze is just cool enough to touch (100–95°F), 10–12 minutes.
Pour half of color one into a large bowl, then pour half of color two into the center, followed by half of color three and half of color four, if using. Repeat with remaining glaze to form concentric circles of different colors in bowl.
Set a chilled, smoothly frosted cake on a cake pan or overturned bowl set inside a rimmed baking sheet or large tray to catch drips. Pour colored glaze over, sweeping back and forth across cake as you pour, to form a marbled pattern. Resist the temptation to touch glaze once it’s poured; it will start to set as soon as it hits the cold cake. Let glaze drip about 10 minutes, then clean up edges with a small offset spatula or butter knife. Transfer cake to a serving platter and chill at least 30 minutes or up to 1 day.
Using a warmed sharp knife, cut cleanly through cake, wiping knife with a warm damp towel between each slice.