Slow-Cooker Flan in a Jar

Ingredients (9)

• 1/2 cup sugar
• 1 tablespoon plus 3 cups hot water (110° to 115°), divided
• 1 cup coconut or whole milk
• 1/3 cup whole milk
• 1/3 cup sweetened condensed milk
• 2 large eggs plus 1 large egg yolk, room temperature, lightly beaten
• Pinch salt
• 1 teaspoon vanilla extract
• 1 teaspoon dark rum, optional