Blend flour, cereal, baking soda, and salt in medium bowl; set aside.
Beat butter and sugars in large bowl with electric mixer at medium speed until creamy, approximately 2 - 3 minutes. Add egg and vanilla. Beat until fluffy.
Add the dry ingredients to the wet ingredients and mix on low speed until combined. Note that the dough will be a bit soft.
Divide the dough into two equal parts. Place each portion on floured parchment or a silicone mat. Dust the top with flour and top with another piece of parchment and roll the "sandwiched" dough into a rectangle of about 1/4-inch thickness.
Remove the top parchment and flip the dough onto a lightly floured surface (I use another piece of parchment). Then, remove the parchment paper that is now on top. Removing the parchment that the dough was rolled in will ensure the dough moves freely when it's time to roll the cookies. NOTE: If the dough will not release from the parchment, place it in the freezer for about 3 minutes and the parchment should peel off easily. Repeat this process with the 2nd dough part.
Make the filling: Mix the butter, sugar and cinnamon together then spread half over each rectangle.
Start at one end, slowly and gently roll each rectangle tightly into a log. If the dough cracks as you roll, simply use your fingers to press the dough to smooth the crack. Wrap the log in parchment or plastic wrap and refrigerate for about 2 hours.
Preheat oven to 375º F and line a baking sheet with parchment paper.
Remove the refrigerated logs and cut into 1/2-inch slices and place on the prepared pan about 2-inches apart.
Bake for 10 - 12 minutes until the cookies are lightly browned on bottom/sides.
Allow the cookies to cool slightly before transferring to a rack to cool completely.
Make the icing by stirring together all ingredients. Drizzle the icing over the cooled cookies, serve and enjoy!
Store cookies in an airtight container.