Preheat the oven to 325°F (163°C) and spray a 15x10x1-inch sheet pan with baking spray.
In a large bowl, use an electric mixer to cream 1 cup plus 2 tablespoons softened salted butter, 6 ounces softened cream cheese, and 2 1/4 cups granulated sugar until light and fluffy.
Add the 6 large eggs one at a time, beating after each addition and scraping down the sides of the bowl as needed.
Beat in 1 teaspoon vanilla extract until combined.
Gradually add 2 1/4 cups cake flour and beat just until blended and smooth; do not overmix.
Pour the batter into the prepared sheet pan and spread evenly with a spatula.
Bake for 30 to 35 minutes, until a toothpick inserted in the center comes out clean. Remove from the oven and let the cake cool in the pan.
To make the icing, combine 1 cup granulated sugar and 1/3 cup evaporated milk in a medium saucepan and bring to a boil over medium heat.
Cover the pan, reduce the heat slightly to prevent boiling over, and simmer with the lid on for 2 minutes.
Remove the saucepan from heat, add 1/2 cup cubed salted butter and 1/2 cup semi-sweet chocolate chips, and stir until melted and smooth.
Let the icing cool slightly, then pour it evenly over the cooled cake. Sprinkle with sprinkles if desired.