Prepare a baking sheet by lining with parchment paper.
Toast pecans in a dry skillet over medium heat for about 5 minutes. Stir frequently to prevent burning. Pecans should be lightly toasted and fragrant.
Clip candy thermometer to medium pot. Combine sugar, brown sugar, cream, and butter over medium heat, stirring regularly until mixture begins to foam and boil.
Once mixture is boiling, stir constantly until mixture reaches 235°F (soft ball stage) on candy thermometer.
Remove from heat and add the toasted pecans and vanilla extract.
Stir vigorously with a rubber spatula for about 3 minutes, or until the mixture begins to thicken.
Pour mixture onto parchment paper and allow to cool. When pralines are completely cool and firm, roughly chop to prepare for fudge.
Prepare a 9-inch square pan by lining with wax paper.
Clip candy thermometer to 3-quart saucepan. Combine condensed milk, whole milk, sugar and butter over low heat, stirring until the sugar dissolves.
Bring mixture to a steady boil for about 20 minutes, stirring frequently and scraping the sides and base of the pan to prevent burning. The mixture should reach 235-245°F (soft ball stage) on candy thermometer.
Remove from the heat and let it cool for a few minutes.
Add chocolate chips and beat the mixture at high speed until it thickens and pulls away from the sides of the pot, about 8-10 minutes.
Reserve some of the chopped pralines for sprinkling on top, and add remaining pralines to the mixture and stir to combine.
Pour fudge into the prepared baking dish and spread evenly. Sprinkle reserved pralines on top and lightly press into the fudge. Let cool completely, about 2 hours, before cutting into 1-inch squares.
Store in an airtight container.