In a small mixing bowl, combine the old fashioned oats with boiling water. Allow to sit for 15-20 minutes before making your cake.
Preheat your oven to 350 degrees F (175 degrees C) and generously coat your 9 x 13 baking pan with butter or non-stick cooking spray.
In a large mixing bowl, cream together your butter with both white and light brown sugar. Pour your vanilla extract into eggs, mix, then add to the soaked oatmeal and combine.
Add the baking soda, cinnamon, and nutmeg (plus optional salt) then combine. Add the flour and mix until thoroughly incorporated.
Add the wet ingredients to your flour mixture and mix just until all of the flour has been incorporated. Transfer to your coated 9 x 13 pan and place on the middle rack of your preheated oven.
Bake at 350 degrees F (175 degrees C) for 30-35 minutes, or until a toothpick inserted into the middle of your oatmeal cake comes out clean (or with a crumb or two on it, but not wet or 'gooey'). Allow cake to cool in the pan while you make the frosting.
Caramel Pecan Frosting
In a medium saucepan, melt your butter at medium heat. Add the light brown sugar, sweetened condensed milk, and vanilla. Stir to combine and until all of the brown sugar has melted.
Whisk frequently while the caramel mixture heats up, then constantly while cooking. Reduce heat as needed to keep your caramel at a low (soft) boil, and just barely bubbling.
Continue to whisk for 5-10 minutes, or until your caramel has thickened and darkened to a golden light brown caramel coloring. It will be soft, creamy, and still pourable (although very thick).
Transfer your caramel mixture to a cool bowl, stir in crushed pecans (reserve some for topping your cake, if desired) and allow to cool 10-15 minutes (but still slightly warm) before spreading over your cake. Finish with additional crushed pecans, if desired.