To make the cake, preheat oven to 350 degrees.
In a large bowl mix flour, baking soda, baking powder, salt, cinnamon.
Add sugar, oil and eggs to flour mixture and mix well to combine.
Next add carrots and pineapple and stir vigorously but do not beat.
Coat a 13 x 9 baking pan with oil and flour. Pour cake batter into pan
Place in heat oven and bake for 30-35 minutes, or until the top springs back when touched. Remove from oven and place on a rack to cool.
To make the frosting place butter and cream cheese in a large bowl and beat on high with an electric mixer for 5 minutes until fluffy.
Next add confectioners’ sugar and vanilla extract and beat on low until combined, then beat on high for 5 minutes until light and fluffy. Spread on cooled cake and sprinkle with chopped walnuts.