In a medium-size bowl, whisk oat flour and cocoa powder together. Then mix in sugar, salt, and baking powder until everything is well incorporated and no lumps of cocoa powder remain. 

Next, mix in the oil and extract  and stir until everything is well incorporated and a thick dough forms, adding water by the tablespoon to create a dough.  You might also need to use your hands to help form the dough.

Form the dough into a ball and place it between two pieces of parchment paper. Press it down with the palm of your hand and then use a rolling pin or can/jar to roll the dough out until it is about ¼-1/8 inch thick. Use a small shot glass or circular cookie cutter (about 1-1.5 inch in diameter) to cut out cookies and place them on your parchment-lined cookie sheet. Gather leftover cookie dough and repeat steps until all dough has been used (should make 30-40 cookies, depending on the size).  Alternatively, you can roll the dough into a log and slice cookies around 1/4 inch thick.  

Bake cookies at 350 F for 12-15 minutes until firm and a little crisp, being careful not to let them burn. Transfer onto a wire cooling rack and let cool completely. Save your parchment paper for the next step!

 While the cookies are cooling, make the chocolate coating by melting chocolate chips, melt in 15-second intervals, stirring in between each to avoid burning. Then melt coconut oil and mix it with the peppermint extract.  Stir in peppermint oil into the melted chocolate, stirring well to keep the chocolate from seizing. 

Using a fork (or your fingers - my method.  Messy on your hands, but quicker), dip the cookies in the melted chocolate. Let any excess drip off or scrape off the side of your bowl before placing them back on your parchment-lined baking sheet. 

Chill thin mint cookies in freezer 10-15 minutes to allow chocolate to set. Enjoy! Store in the refrigerator for a week or in the freezer for up to 3 months.