Wrap a plate in buttered parchment paper.
Melt the sugar in a heavy saucepan over high heat, stirring constantly until sugar becomes thin and no longer clumpy.
When the sugar is completely liquified, add in the butter and watch for the color to change from golden to light brown. Roughly five minutes. If you begin to smell a nutty flavor, remove it from heat.
Add in salt and milk, stirring well.
Reduce heat and simmer for several more minutes until smooth.
Pour caramel onto parchment paper and freeze.
Preheat the oven to 350°F. Cover an 8x8 cake pan in parchment paper and butter it.
Melt the chocolate and butter together in a saucepan until smooth. [You can reuse the caramel pot.]
Remove from heat and add in sugar, salt, and vanilla extract.
Let cool before whisking in eggs, one at a time.
Fold in flour, and pour batter into the prepared pan.
Remove the caramel from the freezer and chop it into squares.
Arrange caramel on top of cake batter, pressing down lightly.
Bake in 350°F oven for 30 minutes.