Combine flaxseed and coffee/water/milk in a large bowl. Set aside to gel while you prepare the sugar syrup.
Add sugar and water to a saucepan and heat over medium low. Whisk constantly until sugar dissolves. Reduce heat if necessary; you don't want the mixture to simmer. Remove from heat and cool for a few minutes until warm but not hot.
To the flax mixture, mix in oil and vanilla. Add sugar syrup and mix until well combined.
Add in almond flour, cocoa powder, corn starch, baking soda, and salt. Mix until a smooth dough forms.
Use a cookie scoop to scoop dough onto a cookie sheet, leaving about 2 inches of space between each cookie. Bake 11 minutes.
Remove cookies from the oven and tap the pan on the counter a few times to deflate the cookies.
Let cookies cool for 2 minutes on the cookie sheet, then transfer to a cooling rack. Let cool completely before serving.
Store in an airtight container. After the first day I like to put mine in the fridge to make them extra fudgy.
Notes
Oil-Free Option: Please note the oil-free cookies are a bit dry on the edges and the dough is more thick (you can use your hands to roll the dough). Replace oil with 3 tbsp (48g) nut butter, and replace sugar with 2 tbsp (30ml) maple syrup + 3 tbsp (36g) sugar. You do not need to dissolve the sugar for the oil-free cookies, so skip step 3 and just add the sweeteners with the rest of the liquid ingredients.