Preheat oven to 350 degrees F. Grease two 9-inch round cake pans (or spray pans with nonstick baking spray), then line bottoms of pans with parchment paper. Grease (or spray with nonstick baking spray) bottom of pan (dust with flour and cocoa powder if desired).
Place butter in a small saucepan over medium-high heat, stir occasionally until melted. Slowly add water, cocoa powder, and coffee into pan and stir constantly until smooth and not grainy, about 3-5 minutes. Remove from heat and cool.
Mix together flour, sugar, baking soda, and 1/4 teaspoon salt in large mixing bowl until well combined. Mix in yogurt, buttermilk, eggs, and 1 teaspoon vanilla until just combined. Mix in cooled chocolate mixture until just combined. Pour batter evenly between two pans.
Bake cakes for about 30-40 minutes or until a wooden toothpick inserted into middle of cake comes out clean. Cool slightly then remove from pans. Cool completely, then cut each cake layer in half horizontally with long serrated knife.
Place chocolate in large mixing bowl. Heat cream in large microwave-safe container in 30 second intervals until hot, about 1-2 minutes. Pour hot cream over chocolate and let stand about 5 minutes. Stir in 1 teaspoon vanilla and 1/8 teaspoon salt, and stir mixture until smooth. Place bowl over ice bath and stir ganache until cold. With an electric mixer on medium speed beat the ganache until just thick enough to spread over cake being careful to not overwhip.
Place one layer of cake on serving platter, top with frosting and spread to form thin layer. Repeat with cake layers and frosting. Frost and decorate top and sides of cake as desired. Garnish sides of cake with Oreo cookie crumbs or as desired. Store cake in refrigerator until ready to serve.
SNAPPY TIPS: Be very careful to not over mix the frosting or you'll end up with chocolate butter rather than a spreadable frosting. The frosting will harden a bit in the fridge.SNAPPY SUBSTITUTIONS: I used dark chocolate cocoa powder for the cake and dark chocolate chips for the frosting. You can substitute different kinds of chocolate but the cake may be a lot sweeter depending on what you use.