These gooey, green-swirled brownies look like they crawled straight out of a Halloween lab.

Every Halloween party needs a little gooey mess! These Monster Slime Brownies are thick and fudgy with bright green marshmallow “slime.” This is one of those desserts that looks straight out of a monster movie but comes together with a box mix and a few kitchen tricks. Perfect for baking with kids, movie nights, or just to add some spooky fun to your weekend treats.

Why You’ll Love This Recipe

• It’s a Halloween treat that’s as fun to make as it is to eat.
• Rich, fudgy brownie base swirled with gooey marshmallow slime.
• Uses a simple box mix, no fancy ingredients needed.
• Customizable with any sprinkles or candies.
• Perfect for parties, bake sales, or a spooky movie night.

Ingredient Notes

• Brownie mix: Any boxed mix works. Choose a fudgy version for the best texture.
• Mini chocolate chips: Add extra richness and a melty bite.
• Marshmallow creme: Fluff gives the slime its soft, gooey texture.
• Green food coloring: Neon gel creates the best spooky glow.
• Halloween sprinkles: Add color, crunch, and a Halloween finish.

Before You Bake

Can I use homemade brownie batter?
Yes. If you prefer homemade brownies, fold in the chocolate chips before adding the slime swirl.

Will the marshmallow melt into the brownies?
It blends in slightly but stays gooey inside. Do not overbake or it will lose its soft texture.

Can I make these ahead of time?
Yes. Store covered at room temperature and the slime will stay soft and chewy.

Monster Slime Brownies

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• Author: Taryn Camp

Ingredients

Scale 1x2x3x

• 1 box of brownie mix (plus ingredients listed on the box: oil, eggs, water)
• 1/4 cup mini chocolate chips
• 1 cup marshmallow creme (Fluff works best)
• 2–3 drops neon green gel food coloring
• Halloween sprinkles

Instructions

1. Preheat your oven according to the brownie mix directions and line a 9×13-inch pan with parchment paper and spray with baking spray.
2. Prepare the brownie batter as directed on the box. Stir in the mini chocolate chips. Pour the batter into the prepared pan.
3. In a small bowl, stir together the marshmallow creme and green food coloring until you get a bright slime color.
4. Dollop spoonfuls of the slime mixture over the brownie batter. Use a butter knife to gently swirl it through, creating gooey streaks of green.
5. Bake according to the box directions (usually 25–30 minutes), until the brownies are set but still fudgy.
6. Remove the brownies from the oven and immediately add sprinkles on top so they stick to the warm brownies.
7. Let cool completely before slicing into squares — the slime will stay soft and gooey inside.

FAQ

How can I make the slime brighter?
Add an extra drop of neon green or a touch of yellow to boost the color.

Can I use melted marshmallows instead of Fluff?
You can, but Fluff holds its texture better once baked.

Can I make these gluten-free?
Yes. Just use a gluten-free brownie mix and the rest of the ingredients are naturally gluten-free.

How much should I swirl the slime?
Only a little. Use a butter knife to create streaks of green without fully mixing it in.

How do I cut them cleanly?
Let the brownies cool completely. Use a warm knife and wipe between each slice.

Sprinkle of Fun

Turn your kitchen into a Monster Lab. Play spooky music, hand everyone a spatula, and let them create their own slime designs. When the brownies come out, name the messiest one the “Mad Scientist Brownie of the Night.”