• Preheat the oven to 350 degrees F. Grease a 9x13 inch baking pan or use an ungreased 9x13 inch disposable foil pan.
• Mix together the flour, baking powder and salt and set aside.
• In the bowl of a stand mixer, beat the butter on medium high speed until light and creamy. Gradually add the sugar, scraping the sides of the bowl once or twice and beating for about 3 minutes or until light and creamy. Beat in the vanilla.
• Reduce mixer speed to low and add eggs one at a time, increasing mixer speed after each egg and beating for 30 seconds. When all the eggs are added, increase speed and beat on high for about a minute.
• Reduce speed to low and add the milk, then with mixer on the very lowest speed add flour and sour cream alternately.
• When flour is fully incorporated, pour mixture into the pan and spread evenly.
• Bake on center rack for about 35 minutes or until cake springs back when touched.

Caramel Frosting

• Mix the softened butter, salt and both brown sugars together in a heavy saucepan. I usually mix them until they are evenly blended just so everything is at the same temperature at the start.
• Place the saucepan over medium heat and just until the mixture begins to boil. Don't let it boil too much or frosting will get too hard. Remove from heat and carefully pour in the milk (it will bubble up) and allow the mixture to return just to a boil. Remove the pan from the heat and add confectioners’ sugar and the vanilla. Beat with a wooden spoon until smooth, then pour over the cake before it starts to set. You may see lumps of confectioners' sugar at this point. Keep beating by hand and let the residual heat from the pot melt the sugar lumps. If it seems to be setting before you can beat out the lumps, set the pot over very low heat as you beat.
• The frosting is ready to pour when all the white spots are gone and it's just barely starting to thicken. Pour over the cake starting in the center and allowing the frosting to flow to the edge.
• Note: I was asked if the cake should be warm or cool. This depends. If the cake is slightly warm the frosting should flow better, but if it is too warm the frosting may slide a little too much and be runny. If the cake is completely cool, the frosting will set quicker. So you can frost the cake while it is still a tad warm or when it is completely cool.