In small bowl, mix oats and boiling water; let stand 20 minutes. Meanwhile, heat oven to 350°F. Grease 13x9-inch pan with shortening or cooking spray; lightly flour.
In large bowl, beat granulated sugar, 1 cup brown sugar and 1/2 cup butter with electric mixer on medium speed until light and fluffy, scraping bowl occasionally. Beat in vanilla and eggs. Beat in oat mixture and remaining cake ingredients until well blended, scraping bowl occasionally. Spread batter evenly in pan.
Bake 35 to 45 minutes or until toothpick inserted in center comes out clean.
Set oven control to broil. In small bowl, beat 2/3 cup brown sugar, 1/4 cup butter and the half-and-half with electric mixer on high speed until smooth. Stir in coconut and nuts. Spoon over warm cake; spread to cover.
Broil 4 to 6 inches from heat 1 to 2 minutes or until bubbly and light golden brown. Cool completely, about 1 hour.