Add the raspberries, sugar, lemon juice, and salt to a small saucepan.

Cook over medium-low heat for about 5–7 minutes, stirring frequently. Gently crush some of the raspberries as they soften.

Mix the cornstarch and cold water in a separate small bowl until completely smooth.

Stir the slurry into the simmering raspberries.

Cook for another 1–2 minutes, stirring constantly, until the mixture is thick and glossy.

Transfer to a shallow bowl and cool completely. The filling should resemble a loose, thick preserve rather than a thin sauce.

Preheat the oven to 350°F (175°C).

Position a rack in the center of the oven.

Line a standard 12-cup muffin pan with cupcake liners.

Whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.

Break apart any lumps of cocoa.

In another bowl, whisk the egg, milk, vegetable oil, and vanilla.

Add the wet ingredients to the dry mixture.

Stir gently until almost combined.

Slowly pour in the hot coffee while stirring gently.

The batter will be relatively thin. This is normal.

Do not continue mixing once the batter is smooth.

Divide the batter evenly between 12 cupcake liners, filling each about two-thirds full.

Bake for 17–20 minutes.

Begin checking at 17 minutes. A toothpick inserted into the cake should come out with a few moist crumbs but no raw batter.

Cool in the pan for about 5 minutes.

Transfer the cupcakes to a wire rack and cool completely.

Simmer about 1/2 cup raspberry puree over medium-low heat until reduced to roughly 3 tablespoons.

For smoother buttercream, strain out the seeds.

Cool the reduction completely before using it.

Beat the softened butter for 3–4 minutes until creamy and lighter in color.

Gradually beat in the powdered sugar.

Add the vanilla and salt.

Beat in 2 tablespoons of cooled raspberry reduction.

Add the remaining raspberry reduction gradually only if needed for flavor and color.

If the buttercream is too thick, add cream or milk 1 teaspoon at a time.

It should hold a defined piping peak.

Once completely cool, remove a small cylinder from the middle of each cupcake.

Make each cavity approximately 1 inch wide and 1 inch deep.

Do not cut through the bottom.

Spoon approximately 1–1 1/2 tablespoons of cooled raspberry filling into each cavity.

Fill close to the top without overflowing.

Transfer the raspberry buttercream to a piping bag fitted with a large star tip.

Pipe a generous, tall swirl over each cupcake.

Top each cupcake with one fresh raspberry.

Finish with irregular curls or small shards of dark chocolate.

For the dramatic lava presentation, cut a cupcake vertically through its center shortly before serving.