In the top of a double boiler set over simmering water or in a heavy saucepan over low heat, melt chocolate, stirring often until chocolate is smooth and reaches 90 degree F.
Meanwhile, on a baking sheet, arrange 24 of the marshmallows in a single layer in blocks of six. Cut the 6 remaining marshmallows into eighths. Place pecans in a small saucepan.
When chocolate is ready, pour about 1/4 cup (enough to cover) over each block of marshmallows. (White streaks in the chocolate mean it's too hot.) Stir about 1/2 cup of the melted chocolate into pecans; keep warm. Stir marshmallow pieces into remaining chocolate. Plug holes between large marshmallows with chocolate-covered marshmallow pieces. Spread pecan-chocolate mixture over top. Let candy stand until set.
To serve, break or cut candy into pieces. Makes 1-3/4 pounds candy.