In a heavy bottomed saucepan, heat milk, heavy cream, sweetened condense milk and sugar, stirring occasionally until the sugar is dissolved and the mixture is hot. (This will take less than 5 minutes – do not bring to a simmer or boil.) Remove from the heat.
Take about a cup of the hot mixture and very slowly drizzle it into the bowl with the egg yolks – constantly whisking while you add the hot mixture. (A slow drizzle will prevent the eggs from cooking as you add the hot cream mixture.)
Once your egg mixture is smooth, slowly pour the egg mixture back into the saucepan with the rest of the hot cream mixture.
Cook the mixture over medium heat, stirring constantly, until the mixture thickens a bit and coats the back of a spoon. (This will take less than 5 minutes as well, and again, do not bring the mixture to a simmer or boil, or it will curdle.)
Remove the pan from the heat and pour the mixture through a fine strainer into a bowl (a large measuring cup like this one works great because you’ll have a spout for pouring the cooled custard into the ice cream maker once it has cooled). Add the Key lime juice and Key lime zest and mix well.
Cover the bowl with a large piece of plastic wrap, pushing the plastic down to touch the top of the custard. (This will prevent a skin from forming on the custard as it cools.) Refrigerate overnight.
Churn in an ice cream maker according to manufacturer’s instructions. After churning, freeze again for 4-6 hours before serving.
While the ice cream is freezing again, prepare the crumb topping by combining all ingredients in a small bowl and mix well. Store in the refrigerator until ready to serve.
When ready to serve, sprinkle the crumb topping over the ice cream and enjoy!
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