In a large bowl, using medium-high speed of an electric mixer, beat the butter and both sugars until light and fluffy. Add the egg and beat just until mixed, then beat in the vanilla, milk powder, baking soda, baking powder, salt and water scraping bowl often.
By hand or using lowest speed of mixer, add flour and stir until it is absorbed. Stir in the oats, flax, sesame, chips and nuts. Empty dough onto a large sheet of waxed paper or plastic wrap and divide in half. Shape each half into a cylinder about 1 ½ to 2 inches in diameter. Chill for several hours or until very firm.
Preheat the oven to 350 F. Line two heavy duty baking sheets with parchment paper or nonstick foil.
Cut the cylinders into ⅓ to ½ inch thick slices and bake at 350 for about 12 to 15 minutes or until the edges are brown. The cookie will be slightly soft at first, but will firm up as they cool.
Using a rounded teaspoon, drop rounds of dough about 2 ½ inches apart onto the parchment or nonstick foil lined baking sheets. Bake one sheet at a time for 10-12 minutes or until edges are nicely browned. Let cool on baking sheets for about 5 minutes, then carefully transfer to a wire rack to finish cooling.