Grease and line a 9-inch square baking pan with parchment paper and set aside. Pre-heat the oven to 350°F (180°C).
In a small mixing bowl, stir together the all-purpose flour, espresso powder (if using), baking soda, and salt and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, mix the butter on medium speed for 1 to 2 minutes, until smooth. Add in the brown sugar and granulated sugar and continue to mix on medium speed for 2 to 3 minutes until, light and fluffy. Add the egg, egg yolk, and vanilla. Mix on medium for another minute until combined. Stop the mixer and scrape down the sides and bottom of the bowl.
With the mixer on low speed, add the dry ingredients in two batches. Mix until just combined and a few steaks of flour still remain. Stop the mixer and fold in the toffee bits and chocolate chunks.
Press the cookie dough into the prepared pan. Smooth out into an even layer with an offset spatula (the dough will be quite thick) as best as possible. Bake in the preheated oven for 31 to 35 minutes or until the center is nearly set. Remove the pan from the oven and press down the edges of the cookie bars with a heat-safe spatula. Allow for the cookie bars to completely cool in the pan on a cooling rack before cutting.
Once completely cooled, use the parchment paper to help remove the cookie bars from the pan. Cut the square in half to create two 9X4 ½-inch rectangles. Cut each half into five triangles with 3-inch bases. There will be 10 triangles total.
To decorate, fill a piping bag fitted with a petal tip (#104) with the Coffee Ganache Frosting. Pointing the narrow end of the petal tip away from your body and toward the top of the cookie bar “tree,” pipe rounded zig-zags to create the swags down the tree. Garnish with sugar pearls, if desired.
Cookie bars are best served within two days of baking. Store leftover in an airtight container at room temperature for up to a few days.