Haven't made this yet but sounds great! I found the recipe here at Our Best Bites blog.
4 juicy limes (try and find ones with thin, smooth skins; they're the juiciest and the thin skin cuts down on the chance of your drink being bitter)
1 c. sugar
6 c. cold water
6 Tbsp. sweetened condensed milk
Mix cold water and sugar very well and chill until ready to use. This step can be done ahead of time.
Wash limes thoroughly with soap (I just use hand-dishwashing soap or regular hand soap); you need the soap to get the wax and pesticides off of the limes because you're using the WHOLE lime, baby. Cut the ends off the limes and then cut each lime into 8ths.
Place 1/2 of the limes in your blender.
Add 1/2 of the sugar water, place the lid on your blender, and pulse 5 times. Place a fine-mesh strainer over a pitcher (the one you'll serve the lemonade in) and pour the blended mixture through the strainer and into the pitcher. Use a spoon to press the rest of the liquid into the pitcher. Dump the pulp and stuff in the strainer into the trash. Repeat with remaining limes and sugar water. Add sweetened condensed milk; DO NOT leave this step out unless you will die of a horrible sweetened condensed milk allergy because this is the secret ingredient! You may want to taste test it at this point; I used giant, thick-skinned limes tonight and didn't test it and it came out a little bitter. If it's bitter, just add some more sugar and maybe a little more milk.
Serve immediately over lots of ice. This does not keep well, so don't make this in advance (although you can cut the limes, mix the sugar water, and measure the sweetened condensed milk in advance). Serves 4, although I can pretty much guarantee you that people will want more; I usually plan on 1 1/2 servings at LEAST per person.