In a large bowl, sprinkle the contents of 4 clear, unflavored gelatin envelopes over 1 cup of cold water. Let it sit for about a minute. The gelatin will absorb the water and swell, a process known as blooming, which ensures it dissolves evenly later.
In a separate bowl, dissolve 2 tablespoons of instant coffee and ¼ cup sugar (adjust to taste) in 2½ cups of boiling water.NOTE: I've used ½ cup less water than the package recommends for a slightly firmer texture. Feel free to adjust the water amount to your preference and refer to the gelatin package for guidance.
Pour the hot coffee mixture over the bloomed gelatin. Whisk continuously until the gelatin is fully dissolved and the mixture is smooth.PRO TIP: Boiling water is key to dissolving the gelatin properly. If the gelatin hasn't fully dissolved, briefly microwaving the mixture for about 30 seconds should do the trick.
Pour the mixture into a baking dish (I used an 8x8-inch) or into small cups, depending on how you'd like to present it. Let it cool to room temperature. Cover and refrigerate until it sets, about 3-4 hours. Once set, cut the gelatin into cubes.
In a bowl, combine heavy cream, condensed milk, and a tablespoon of coffee liqueur or a teaspoon of vanilla extract. Lightly whisk just until it thickens and has slightly increased in volume, but stop before it reaches soft peaks to maintain a pourable consistency.
To assemble, place the coffee gelatin cubes in serving glasses or a serving dish. Pour the cream mixture over the gelatin cubes. Chill until ready to serve.
Notes
Unflavored gelatin: For the coffee jello, I used 4 envelopes (0.25-ounce or 2 ½ teaspoons each) of Knox gelatin powder (clear and granulated). Feel free to use another brand or substitute with gulaman or agar agar powder, which sets a bit firmer than traditional gelatin.
Instant coffee: I used instant coffee granules which quickly dissolves in hot water. If you prefer, you can use espresso powder or freshly brewed coffee. Adjust the amount to your taste.
Heavy cream: I used heavy cream for a rich and creamy texture. You can use whipped cream for a fluffier texture, light cream for a ligher version, or another dairy of your choice.