Preheat oven to 350F (175C) and prepare two 9" round cake pans by lightly greasing and flouring I also recommend lining the bottom of each pan with a round of parchment paper. Set aside.
Combine flour, sugars, cocoa powder, instant coffee, baking soda, and salt in a large bowl (or in the bowl of a stand mixer) and use an electric mixer to stir until well-combined.
Add oil and melted butter and stir well (mixture will be very thick), then stir in eggs, egg yolk, and vanilla extract.
Gradually add buttermilk, stirring until completely combined.
Slowly add hot water (be careful not to splash yourself!), stirring until ingredients are well-combined. Pause occasionally to scrape down the sides and bottom of the bowl with a spatula to ensure all ingredients are thoroughly combined.
Evenly divide batter into prepared pans. Transfer to oven and bake on 350F (175C) for 25-30 minutes and a toothpick inserted in the center comes out clean or with a few moist crumbs. Allow cakes to cool for 15 minutes before inverting onto cooling rack to cool completely before filling and frosting.
Once your cake is in the oven, begin preparing your coconut/caramel German Chocolate Cake Filling. In a medium-sized saucepan, combine egg yolks and evaporated milk and whisk until eggs are lightly beaten.
Add butter, sugars, and salt and place saucepan over medium/low heat (keep heat low so you do not cook your eggs!). Stir constantly until mixture is a light golden brown and thickened (about 10 minutes). Remove from heat and stir in vanilla extract, chopped pecans, and shredded coconut.
Allow to cool completely before spreading over cake.
In the bowl of a stand mixer OR in a large bowl using an electric mixer, beat butter until creamy. Gradually add powdered sugar and cocoa powder until completely combined. Stir in vanilla extract and milk.
Once your cake layers have cooled and your coconut/caramel filling is no longer warm to the touch, you can assemble your German Chocolate Cake!
Place one cake round on serving tray or cake platter. Use your chocolate frosting to spread or pipe a border around the edge of the cake. Fill the border with half of your coconut/caramel filling.
Top with second cake round. Frost top and sides of entire cake with remaining chocolate frosting.
Top off cake with remaining half of coconut/caramel frosting.
Slice, serve, and enjoy!