Preheat the oven to 350°F (175°C). Grease and line two 8-inch cake pans.

In a bowl, whisk together flour, baking powder, and salt.

In a separate bowl, cream butter and sugar until light and fluffy.

Beat in eggs one at a time, then add vanilla extract.

Gradually add dry ingredients alternately with milk, mixing until combined.

Pour batter into prepared pans and bake for 30-35 minutes or until a toothpick comes out clean.

Let the cakes cool completely.

Beat butter until creamy. Gradually add powdered sugar, mixing until smooth.

Add heavy cream and vanilla, then beat for another 2-3 minutes until fluffy.

Melt isomalt according to package instructions and pour it onto a silicone mat to cool. Break into crystal-like shards once set.

If using rock candy, sort crystals by color.

Color fondant with gel food coloring to create a marbled stone effect.

Level the cake layers if needed. Spread buttercream frosting between layers and stack them.

Carve out a section of the cake to create a geode cavity. Frost the entire cake and chill for 30 minutes.

Roll out the marbled fondant and cover the cake smoothly. Trim excess fondant.

Apply edible crystals into the carved cavity, starting with darker colors inside and lighter colors outside.

Paint the edges of the cavity with metallic edible paint for a polished look.

Add shading and highlights using food coloring or dusts.

Present your cake on a decorative stand for maximum impact!