In a small bowl, whisk together oats, flour, baking powder, cinnamon, and salt. Set aside.
In the bowl of a standing mixer fitted with the paddle attachment, beat butter and sugar until light and fluffy, 2 to 3 minutes.
Add egg and vanilla and beat until evenly combined.
Add oat mixture and mix on low just until combined. (Do not overmix.)
Refrigerate dough for 1 hour.
When ready to bake, preheat oven to 325°F / 163°C.
Line two large sheet pans with parchment paper.
Use a teaspoon to measure out heaping 1 teaspoon portions of dough. Roll the dough into balls and arrange on prepared sheet pans, leaving about 3 inches of space between them.
Bake until edges are golden and centers are set, rotating the pans halfway through cooking, 9 to 12 minutes. (Note: Keep a close eye on these and remove them when they are flat and the edges are deep golden brown.)
When cookies are done baking, carefully lift the parchment paper with the cookies off the pan and set on a countertop to cool completely.
While they cool, melt chocolate in the microwave, stirring occasionally to avoid burning.
Use a spoon to drizzle the cookies with melted chocolate. Sprinkle with some salt, if using.
Give the cookies a couple of hours to cool completely and for the chocolate to firm up. You can put them in the refrigerator to speed things along.
Cookies can be stored in an airtight container at room temperature for up to 3 days.