In a bowl, combine flour, baking soda, baking powder and salt. Mix to combine.
In the bowl of an electric mixer, beat butter and sugar using the paddle attachment until creamy. You can also mix by hand with a wooden spoon or use a handheld electric mixer.
Add flour combo and mix to combine.
Add ½ the egg and milk. Mix until thoroughly combined.
Add the nut meal/ nut flour and mix until thoroughly combined.
It is a dry looking dough, but do not be tempted to add more liquid. The heat from your hands will provide the additional moisture to make the cookies come together.
Place a piece of parchment paper on a cookie sheet. Spray with non stick spray.
Form walnut sized balls. Make an indent and place a walnut piece in the center. Place on prepared sheet. Repeat until all the cookies have been formed. Leave to rest for 15 minutes.
While the cookies are resting, preheat the oven to 350 F. Brush cookies with the other half of the egg.
Bake for 15-20 minutes, until golden brown. As soon as you take them out, press the walnut cookies into the dough again if necessary. Some may have risen a bit. Leave on the cookie sheet for 10 minutes to set.