Preheat oven to 350°F. Grease and flour a 9x13 inch baking dish.

Pour boiling water over the oats in a small bowl. Stir together and let cool.

In a large bowl, cream together the softened butter, brown sugar, and sugar until light and fluffy. Beat in the eggs one at a time.

In a separate medium bowl, whisk together the flour, baking soda, cinnamon, and salt.

Gradually add the dry ingredients to the wet ingredients, alternating with the cooled oat mixture. Mix until just combined. Do not overmix.

Spread the batter evenly into the prepared baking dish.

Bake for 45 minutes, or until a toothpick inserted into the center comes out clean.

While the cake is baking, prepare the topping. In a medium bowl, stir together the melted butter, brown sugar, shredded coconut, chopped walnuts, cream, and vanilla extract until well combined.

Once the cake is baked, remove it from the oven. Spread the topping mixture evenly over the hot cake.

Return the cake to the oven and broil for about 2 minutes, watching carefully, until the topping is melted and bubbly.