In a large bowl or bowl of a stand mixer, combine butter, Swerve Granular, and Swerve Brown. When adding the Swerve Brown, sprinkle it on top of the other ingredients to make sure it evenly distributes when mixing.
Mix on medium-high for 5-6 minutes until fluffy.
Then, add one egg and mix it in on low. Add the second egg, then the third, and do the same.
Add lemon extract, lemon juice, and lemon zest, scrape down the sides of the bowl, and mix on low until well combined.
In a small bowl, thoroughly combine the coconut flour, oat fiber (if using), baking soda, and salt. Add the dry ingredients to the wet and mix in on low for 30 seconds until combined.
Lightly spray the bottom of a casserole dish (I used 7x11”) with non-stick cooking spray, then evenly spread the batter into the dish.
Place casserole dish in the oven and bake for 15-22 minutes (highly dependent on your oven), until the edges begin to lightly brown. Do NOT overbake! The middle of the casserole will still be relatively soft/wet, but an inserted toothpick should come out mostly clean. The cookie bars will firm up as they cool.
Place the dish on a wire rack and let the cookie bars cool completely. (For quicker cooling, after 30 minutes you can place the dish in the fridge.)
Frosting Instructions
Add frosting ingredients to a microwave-safe bowl and beat on medium-high for 5 minutes, making sure to scrape down the sides during the process.
Add bowl to the microwave and microwave for 10 seconds. Remove from the microwave and stir to distribute the heat. NOTE: This step ensures that the frosting will be spreadable. If it’s left to its usual texture, the top of the cookie bar will lift up and mix into the frosting.
Spread the frosting evenly onto the top of the cookie bar. (If you have any trouble spreading it, microwave the frosting for a few more seconds.)
Refrigerate the cookie bar for at least 30 minutes to return the frosting back to its regular texture.
Decorate each cookie with a thinly sliced lemon wedge, if desired.