Preheat oven to 350 degrees.
Take an 8x8 pan, line with parchment and coat with nonstick spray. Set aside
In a large bowl, beat together butter, brown sugar, and vanilla until fluffy.
Beat in eggs, then flour, cinnamon, baking powder, and salt.
Continue to mix until well blended.
Fold in carrots, walnuts or pecans, and raisins (if using), then mix in milk.
Gently stir until fully incorporated.
Pour into prepared 8x8 inch pan.
Bake for 30-35 minutes or until middle is set and a toothpick comes out clean.
Remove from oven and let cool at room temperature for 30 minutes before icing.
Make icing by beating together cream cheese, butter, vanilla, powdered sugar and milk until thick and fluffy.
Spread over carrot cake bars, and sprinkle with walnuts or pecans.
Cut into squares and enjoy.