pre-heat oven to 350˚

mix flour, oats, brown sugar, cinnamon, baking soda, and salt together

cut cold butter in small pieces and work into the flour mixture until it looks like crumbs, be sure you don't leave any big chunks of butter

set aside one cup of the crumbs, press the rest in a 9X13 baking pan

drain the pineapple and save the juice, no need to drain it completely you just want most of the juice drained off

in a saucepan over medium heat lightly crush the pineapple with a potato masher, alternately you can chop about half of it with some scissors, you just want it a little broken up

let it cook while grate the carrots, then add the carrots to the pan, stir in the sugar and let it cook for 5 minutes, stirring occasionally, if it's too dry add a splash of the pineapple juice

measure 1/2 cup of the pineapple juice and whisk it into the flour until smooth

pour the pineapple juice flour mixture into the saucepan on the stove, cook and stir until the mixture is thick and bubbly

the pour the bubbling carrot-pineapple mixture over the base layer, spreading evenly

sprinkle with reserved crumbs

place in preheated oven and bake for 35-40 minutes or until set and firm

cool 

mix the softened cream cheese and sugar together

add milk a teaspoon at a time until it's drizzling consistency either for using a bag OR using a fork and bowl method

put in a piping bag or use a fork and drizzle over the cooled carrot cake bars