Preheat oven to 350 degrees
Spray bottom and sides of a 9x13 inch casserole dish or cake pan with non-stick spray. I, then, like to line it with heavy duty aluminum foil allowing foil to extend over edges.
In a bowl, whisk flour, baking powder, salt and cinnamon.
In the bowl of a stand mixer, cream butter and brown sugar at medium until light and fluffy, about 3 minutes. Scrape sides of bowl.
Add the eggs, one at a time, beating until well combined.
Add vanilla and beat until combined.
Add the dry ingredients and beat at low speed until just combined, about 30 seconds. Scrape sides of bowl as needed.
Fold in apple and toffee.
Spread batter into prepared pan. Bake 15 to 20 minutes or until set in the center. Test by inserting a wooden pick in center. Cake is done when pick comes out clean.
Place on a wire rack to cool.
Prepare frosting while cake cools.