Lightly grease a 9x13" baking dish or line with parchment paper or foil, leaving an overhang on all sides to lift them out for easy cutting later. Set aside.
In a medium bowl, whisk together flour, cinnamon, baking soda, and salt; set aside.
Cream butter and sugar on medium-high speed until smooth; beat in egg and vanilla. Beat in pumpkin puree (mixture may appear curdled).
Reduce speed to low, and add dry ingredients (including flax seed if using). Mix on low until just combined. Fold in chocolate chips and toffee bits.
Spread batter evenly in prepared pan. If desired, sprinkle with cinnamon sugar topping. Otherwise, leave as is.
Bake until edges begin to pull away from sides of pan and a toothpick inserted in center comes out with just a few moist crumbs attached, 35 to 40 minutes. Cool completely in pan on cooling rack.
When cool, cut into 24 squares.
Store in an airtight container at room temperature for up to a week, or freeze for longer storage.
Recipe by Hidden Ponies at https://hiddenponies.com/2012/09/saturday-sweets-pumpkin-chocolate-toffee-coffee-cake/