Coffee-Toffee Crunch Cake

Name: Coffee-Toffee Crunch Cake
Prep time: 1 h
Total time: 1 h
Cuisine: American
Ingredients: Cooking spray, for the pans; 3 c. all-purpose flour, plus more for the pans; 3 c. granulated sugar; 1 tsp. baking powder; 1 tsp. baking soda; 1/2 tsp. kosher salt; 3 sticks salted butter; 1/3 c. instant coffee granules; 1 1/2 c. boiling water; 3/4 c. buttermilk; 3 large eggs; 1 tbsp. vanilla extract; 2 1/2 sticks salted butter, at room temperature; 1 1-pound box powdered sugar; 1/4 c. instant coffee granules; 3/4 c. heavy cream, plus more if needed; 4 chocolate-covered toffee bars (such as Heath)