Preheat oven to 350 degrees Fahrenheit. Grease and flour 9 x 13-inch pan.

In a medium bowl, whisk together flour, salt, baking powder, cinnamon, and nutmeg.

Combine margarine and sugars in a large mixing bowl. Using an electric mixer, beat until light and fluffy. Reduce speed to low and add eggs, one at a time. Mix until smooth.

Add the flour mixture slowly, until flour is just incorporated. Stir in diced apples. The dough will be thick, similar to cookie dough. Don’t worry if it is hard to mix.

Spread dough evenly in prepared pan. (You can flatten the dough with damp hands.) Bake for 30-35 minutes. Allow to cool completely.

Combine powdered sugar, hot water, and vanilla extract in a Ziploc bag. Mix until smooth. It should be the consistency of glue; if it is too thick, add more water one teaspoon at a time. If it is too thin, add more powdered sugar.

Snip off a corner of the bag and drizzle over cooled bars.