MAKE THE COOKIES
Heat the oven to 375 and line two large baking sheets with parchment paper or non-stick baking mats.
In the bowl of a stand mixer with paddle attachment, beat the butter on medium speed for 2 minutes. (Alternatively, use a medium-sized mixing bowl and a hand mixer.)
Add the sugar and beat for 2 more minutes, scraping down the sides as needed. Add the egg and vanilla and mix to combine for another 30 seconds.
Add the flour, baking powder and salt and beat on low for 1 1/2-2 minutes until the dough starts to come together. Push the dough and scraps together to form a ball with your hands.
Turn the dough out on a lightly floured flat surface and divide the dough in half with a knife.
Working with one half at a time, pat the dough into a disc with your hand, then roll the dough out with a rolling pin to 1/4 inch thickness.
Cut the dough with the cookie cutter, doing one cut out at a time, gently lifting each cookie and placing on the prepare baking sheets.
Repeat with the other portion of dough, pressing the leftover scraps together and rolling them out for cutting too, so as not to waste any dough.
Refrigerate the sheets of unbaked cookies for 20 minutes. Remove from the fridge and bake for 8-11 minutes until barely golden brown on the very edges. The length of cooking time will depend on your oven, so check at 8 minutes for soft cookies..baking for 11 minutes or slightly more will make the cookies crisper (but very good!)
Cool the cookies on baking racks.
MELT THE CHOCOLATE
Place some water in a medium-sized saucepan on the stove-top, and place a heat-proof glass bowl fitted on top. The sauce pan only needs an inch or two of water on the bottom, and you don't want the glass bowl to touch the water.
Add the white chocolate, and simmer until melted.
Let the chocolate cool for a few minutes, then transfer it to a plastic freezer bag. Push all the chocolate in one corner of the freezer bag, then snip off the very end with scissors and squiggle a little white chocolate on each cookie. Alternatively, you could drizzle the chocolate with a fork or spoon.
Sprinkle with dye-free sprinkles or decorative sugars.
Let the cookies set completely, about an hour.