Mix the Dairy - To a blender, add sweetened condensed milk, heavy whipping cream, chocolate syrup, and vanilla extract. Mix on low speed until just combined, about 20 to 30 seconds. You do not want the cream to thicken.
Add the Whiskey - Gradually add the whiskey with the mixer running on low until just combined, about 20 seconds.
Storing Irish Cream - Transfer Irish cream to glass storage bottles. Cover and refrigerate for up to 2 weeks. Gently shake the bottle before using it.
Notes
Recipe Yield: 32 ounces (960 ml)
Serving Size: 1.5 ounces (45 ml)
Bowl Method: In a large bowl, whisk the sweetened condensed milk, heavy whipping cream, chocolate syrup, and vanilla extract until thoroughly combined but not too frothy.
Mixer Method: If using a stand mixer, use the whisk attachment and mix on low speed (setting 2) for about 30 seconds. Gradually whisk the whisky into the cream mixture until combined, about 20 to 30 seconds.
For a Coffee Flavor: Add 1 to 2 teaspoons of instant coffee or instant espresso when blending the cream mixture.
Using Cocoa Powder: Substitute chocolate syrup with ½ to ¾ teaspoon cocoa powder. The mixture will be slightly less sweet. Whisk in the cream mixture until fully dissolved, then add the whiskey. Strain if needed to remove any clumps.
Stronger Chocolate Flavor: Add 1 to 2 tablespoons of chocolate syrup. This also makes it slightly sweeter with less of an alcohol bite.
Sweetener Substitutes: Instead of sweetened condensed milk, use 1 ⅓ cups granulated or brown sugar. Fully dissolve in the cream. You may need more cream if the liqueur is too thick after adding the alcohol. Use equal parts of honey or pure maple syrup. The consistency will not be as dense with these alternatives since there are no milk solids.
Make it Dairy-Free: Substitute heavy cream with canned unsweetened coconut milk. Use one of the sweetener substitutes above instead of sweetened condensed milk. The taste will be different than the original recipe.