Step 1: In a medium saucepan, combine heavy cream, whole milk, granulated sugar, and instant espresso powder (if using powder). If using liquid espresso, it will be added later.
Step 2: Heat the mixture over medium heat, stirring occasionally, until the sugar is completely dissolved and the mixture just begins to simmer around the edges. Do not boil.
Step 3: Remove from heat. If using liquid espresso or strong coffee, stir it in now along with the vanilla extract. Whisk thoroughly to combine.
Step 4: Transfer the coffee base to a bowl or airtight container. Cover and refrigerate for at least 4 hours, or preferably overnight, until thoroughly chilled. This step is crucial for the best texture.
Step 5: For traditional churned ice cream: Pour the chilled coffee base into your pre-frozen ice cream maker bowl. Churn according to the manufacturer's instructions (usually 20-30 minutes) until it reaches a soft-serve consistency.
Step 6: For no-churn ice cream: In a large bowl, combine the chilled coffee base with the sweetened condensed milk and vanilla extract. Whisk until fully blended.
Step 7: In a separate, chilled bowl, whip 2 cups of very cold heavy cream until stiff peaks form. Gently fold about one-third of the whipped cream into the coffee-condensed milk mixture to lighten it, then carefully fold in the remaining whipped cream in two additions until just combined, being careful not to deflate the mixture.
Step 8: Transfer the churned or no-churn mixture to an airtight freezer-safe container. Press a piece of parchment paper or plastic wrap directly onto the surface to prevent ice crystals.
Step 9: Freeze for at least 4-6 hours (churned) or 6 hours to overnight (no-churn), until firm. Let sit at room temperature for 5-10 minutes before scooping and serving.