Heat oven to 400° F.
Melt the butter and chocolate in a pan over low heat.
Using a mixer, beat the eggs and sugar until pale yellow and fluffy. Lower speed and pour in the melted chocolate and vanilla. Mix in the flour and 1/4 teaspoon salt. Fold in the peanut butter cups. Scrape into a buttered 9-by-13-inch baking dish. Bake until the tip of a knife comes out clean, about 35 minutes.