Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.
Whisk together the flour, baking soda, salt, and cinnamon (if using) in a medium bowl. Set aside.
Using an electric mixer on medium speed, beat the butter and sugar in a large bowl until light and fluffy (2 to 3 minutes). Mix in the egg and vanilla until well combined.
With the mixer on low speed, gradually add the flour mixture to the butter mixture, mixing just until combined.
Scoop the dough into 1-tablespoon portions. Roll into balls and place on baking sheets. Using your fingers or the bottom of a glass, slightly flatten each dough ball to 1 3/4 to 2 inches in diameter.
Press 5 almond slices into the top of each cookie to resemble the petal shape of a sand dollar. Sprinkle with cinnamon-sugar.
Bake one pan at a time for 10–12 minutes, or until edges are set and bottoms are just golden.
Cool on the pan for about 5 minutes before transferring to a wire rack to cool completely.
