For cookies: Sift first three ingredients in a medium bowl. Use a mixer and beat Horio cow butter and 1 cup of sugar in a large bowl.
Add egg, sour cream and vanilla and mix for 1 minute. Then, slowly mix solid ingredients until blended.
Unite mixtures, knead and separate in two doughs. Press to flatten and round, cover with film and refrigerate for 1 hour (this step can be prepared a day ahead and keep in the fridge. The next day, leave doughs in room temperature to soften).
Spread parchment paper on two baking sheets. On a clean surface, sprinkle some sugar and then sprinkle two doughs, too. Roll them with a rolling pin until 0,50 cm thick. Make cookies with a cookie cutter.
Transfer cookies in baking sheets layered with parchment paper, leaving a 2 cm distance between them. Use all dough. Leave cookies in sheets for 15 minutes to 1 hour.
Preheat oven at 175C. Bake cookies, one sheet at a time, for 12 minutes – until brown on their edges. Transfer to a rack and let cool completely.
For glaze: In a medium bowl put powdered sugar, milk and vanilla. Mix well until soft and elastic, adding spoons of milk if mixture is too thick and more sugar if mixture is too thin. Use a knife or a spatula to spread glaze over cookies.
If using colorful truffles, sprinkle cookies before glaze cools off.
Glaze can be made three days ahead. Keep wrapped in parchment paper and in airtight container.