Right click on the sketch and select 'Copy Sketch'. Then, in the new part studio, start a new sketch, right-click on the sketch, and select 'Paste into sketch'
Check out Sketch a Cartoon Character for inspiration for your cookie cutter sketch!
Select the sketch, then select 'Offset'. Drag the arrow out or double click on the dimension to change it.
Select any sketches that you want to make an indentation into the cookie (like the eyes on the smiley face!), and click 'Extrude'. Decide how deep you want the indentation by setting the 'Depth'. Then, select the outer edges and 'Extrude' them with a larger 'Depth' to cut through the cookie!