For the cake:
Preheat the oven to 350-degrees. Mix the rolled oats with the butter and hot water in a large heatproof container. Set aside for about 20 minutes.
In a medium bowl, whisk together the flour, kosher salt, baking soda, cinnamon, and nutmeg.
In a large mixing bowl, blend the sugars together with the eggs and the vanilla. Beat on medium speed for about 2 minutes until very light in color. Scrape down the sides and bottom of the bowl and beat for another 30 seconds.
Scoop the flour mixture into the sugar and eggs, adding about ⅓ of it at a time, beating in on low speed just until combined.
Stir the oatmeal, water, and butter into the batter blending in using low speed, about 1 minute.
Add the raisins, coconut, and pecans mixing in on low speed. The batter will be somewhat thinner than other cake batters.
Pour the batter into a 13” x 9” baking dish that has been generously sprayed with a cooking spray or greased and floured.
Bake in a 350-degree oven for 30 to 35 minutes or until a toothpick inserted into the center of the cake comes out clean.
Cool the cake on a wire rack while you make the frosting.
For the frosting:
Combine the butter, half and half, and sugar in a medium saucepan. Cook over medium-low heat, stirring frequently until the sugar is dissolved, about 5 minutes.
Raise the heat to medium and bring the frosting to a boil. Remove the pan from the heat and add the vanilla, pecans, and coconut.
Beat the frosting on medium speed until it thickens, about 2 minutes. Spread the warm frosting over the cake.