In a medium bowl, melt butter. add vanilla extract and instant coffee. Mix until evenly combined but not too much as to aerate the butter.
Next add these content to a large bowl. Add the brown sugar and granulated sugar. Mix. Add the eggs and beat until well mixed.
Finally add the dry ingredients: flour, baking soda, corn starch, and salt. Mix well until a cookie dough is formed.
Preheat oven to 350°F. Line a square baking dish with parchment paper and place the dough. Spread it out with a spatula.
Bake in the oven for 30-35 minutes or until an inserted toothpick does not lift any dough.
Remove and let cool. In a small bowl mix speculoos butter with crushed speculoos cookies. Spread this on top the cookie bars. Slice and serve!
