Preheat oven to 350°F. Butter a 13”x9” rectangle pan.
For the oatmeal mixture, pour 1 ⅓ cups boiling water over quick oats, and stir until well combined. Set aside to cool.
For the dry ingredients, in a medium bowl, whisk together flour, cinnamon, baking soda, and salt.
For the wet ingredients, in a stand mixer or large mixing bowl with an hand electric mixer, cream together the butter, brown sugar, and white sugar until light and fluffy. Beat in the eggs one at a time.
Add the dry ingredients to the mixer bowl, and blend for 1 minute.
Add the oat mixture 2 tablespoons at a time, and blend an additional minute.
Pour the batter into the prepared pan. Bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
When 10 minutes remain on the cake, start making the topping. Place a medium saucepan over medium heat and add brown sugar, butter and milk. Cook until the brown sugar is dissolved and mixture comes to a boil. Turn off the heat and stir in vanilla, coconut, and pecans.
Spread the frosting in an even layer on top of the cake. Optional step, place the cake under the broiler for 1-3 minutes, or until the topping is bubbly and lightly browned. Watch carefully to avoid burning.
Allow the cake to cool slightly before serving. Optional, serve with fresh whipped cream or ice cream.