Preheat and prepare the pan.
Preheat your oven to the temperature specified in your brownie recipe or box (typically 350 °F / 175 °C). Grease or line a 9×13 baking pan with parchment paper or nonstick spray, leaving some overhang for easy removal.

Pour half the brownie batter.
Pour approximately half of the prepared brownie batter into the bottom of the pan and spread it evenly to cover the entire base.

Place peppermint patties.
Arrange 32 miniature chocolate peppermint patties evenly on top of the first layer of brownie batter. You can do 4 rows × 8 columns or evenly spaced, making sure none touch the edges too closely. The patties will become centers of mint surprise.

Cover with remaining batter.
Carefully spoon or pour the remaining half of the brownie batter over the peppermint patties, gently spreading it to fully cover them. Don’t press too hard — you want to preserve the integrity of the patties, but ensure they are well covered.

Bake the brownies.
Bake in the preheated oven for the time recommended by your brownie recipe or box instructions, plus an extra 5–10 minutes as needed (depending on how thick your layers are). Insert a toothpick near the edge (not into a patty) to test doneness; it should come out mostly clean with a few moist crumbs but not wet batter.

Cool completely.
Allow the brownies to cool fully in the pan on a wire rack. Cooling is important so that the brownie structure firms up and so cutting later is neater.

Cut into squares.
Once cooled, use the parchment overhang (if used) to lift the brownie slab from the pan. Place on a cutting board and cut into 32 roughly equal squares, so that each square has one embedded peppermint patty.

Decorate with candy eyes and gel.
For each brownie square, attach candy eyes (two per brownie) using a small dab of the white decorating gel as “glue.”
Use the red and white decorating gel tubes to draw “fangs,” “blood drips,” or other vampire-style accents (for instance, red gel dripping from corners, white gel to outline lips or teeth). Let the gel set for several minutes before serving.