Making the Oatmeal Cookie Bar Base:

1. Preheat and Prepare: Preheat your oven to 350°F (175°C). Grease and flour a 9×13 inch baking pan, or line it with parchment paper, leaving an overhang on two sides for easy removal. This overhang will act as “handles” later.
2. Cream Butter and Sugars: In a large mixing bowl, using an electric mixer (stand mixer with paddle attachment or hand mixer), beat the softened unsalted butter, packed light brown sugar, and granulated sugar together on medium speed until light and fluffy. This usually takes about 2-3 minutes. Scrape down the sides of the bowl occasionally. This step is crucial for incorporating air, which contributes to the texture of the bars.
3. Add Eggs and Vanilla: Add the large eggs one at a time, beating well after each addition until fully incorporated. Then, beat in the vanilla extract. The mixture should be smooth and well combined.
4. Combine Dry Ingredients: In a separate medium bowl, whisk together the all-purpose flour, baking soda, ground cinnamon, ground nutmeg (if using), and salt. This ensures the leavening agents and spices are evenly distributed.
5. Mix Wet and Dry: Gradually add the dry ingredient mixture to the wet ingredient mixture, mixing on low speed until just combined. Be careful not to overmix at this stage; overmixing can lead to tough bars.
6. Incorporate Oats: Gently fold in the old-fashioned rolled oats with a spatula until they are evenly distributed throughout the dough. The dough will be thick and a bit sticky.
7. Press into Pan: Transfer the oatmeal cookie dough to your prepared 9×13 inch baking pan. Using your hands (lightly greased or dampened with water helps) or the back of a spatula, press the dough evenly into the bottom of the pan. Try to get it as level as possible for even baking.
8. Bake to Perfection: Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with moist crumbs attached (not wet batter). Be careful not to overbake, as this can result in dry bars.
9. Cool Completely: Remove the pan from the oven and place it on a wire rack to cool completely. This is very important! If you try to ice the bars while they are still warm, the icing will melt and become a runny mess. Cooling can take 1-2 hours.

Making and Applying the Vanilla Icing:

1. Prepare the Icing: Once the bars are completely cool, prepare the icing. In a medium bowl, whisk together the sifted powdered sugar, 3 tablespoons of milk (or cream), vanilla extract, and the optional pinch of salt.
2. Adjust Consistency: If the icing is too thick, add more milk, one teaspoon at a time, until it reaches your desired pouring or spreading consistency. It should be thick enough to coat but still spreadable. If it becomes too thin, add a little more sifted powdered sugar.
3. Ice the Bars: Pour the vanilla icing evenly over the cooled oatmeal cookie bars. Use an offset spatula or the back of a spoon to spread it smoothly to all edges.
4. Let the Icing Set: Allow the icing to set completely before cutting into bars. This can take about 30 minutes to an hour at room temperature, or you can speed it up slightly by placing the pan in the refrigerator for 15-20 minutes.
5. Cut and Serve: Once the icing is set, use the parchment paper overhang to lift the bars out of the pan onto a cutting board. Cut into squares or rectangles of your desired size. A sharp knife, wiped clean between cuts, will give you the neatest edges.