In a deep bowl, first whisk the butter, then add the sugar. After 5 minutes of beating, add the egg and continue to beat until the whole mixture is homogeneous. Add the vanilla extract and stir. In a deep bowl, mix together the flour, cocoa, baking soda and salt. Gradually add the butter from the dry ingredients and the milk, stirring between each addition, until you have used up the ingredients. Using a scoop of ice cream, apply the mixture to pans lined with paper, but spread about 5-8 cm apart as they spread during baking. Bake in a preheated oven at 170 degrees for about 10-12 minutes. Allow to cool completely. For the filling: using a mixer, whisk all the ingredients together until you get a nice thick filling and apply it on a cookie. Cover with another cookie to form sandwich cookies. Garnish with powdered sugar and lemon zest if desired. Optionally, use food coloring in the glaze.