Define buildconfigfield for an specific flavor AND buildType

For your specific case, you can also just play with defaultConfig:

defaultConfig {
    buildConfigField BOOLEAN, VARIABLE, TRUE
}

buildTypes {
    debug {
        buildConfigField BOOLEAN, VARIABLE, FALSE
    }
    release {
    }
}

productFlavors {
    VANILLA {
    }
    CHOCOLATE {
        buildConfigField BOOLEAN, VARIABLE, FALSE
    }
}

The Default value is TRUE, but then you put FALSE to all Debug builds and all Chocolate builds. So the only remaining TRUE is VANILLA-release.

Here is a solution without lacks I've described under Simas answer

buildTypes {
    debug {}
    release {}
}

productFlavors {
    vanilla {
        ext {
            variable = [debug: "vanilla-debug value", release: "vanilla-release value"]
        }
    }
    chocolate {
        ext {
            variable = [debug: "chocolate-debug value", release: "chocolate-release value"]
        }
    }
}

applicationVariants.all { variant ->
    def flavor = variant.productFlavors[0]
    variant.buildConfigField "boolean", "VARIABLE", "\"${flavor.variable[variant.buildType.name]}\""
}

Within the Gradle build system, buildTypes and productFlavors are unfortunately two separate entities.

As far as I am aware, to complete what you want to achieve, you would need to create another build flavour as such:

buildTypes {
        debug{}
        release {}
    }

    productFlavors {
        vanillaDebug {
             buildConfigField BOOLEAN, VARIABLE, FALSE
        }
        vanillaRelease {
             buildConfigField BOOLEAN, VARIABLE, TRUE
        }
        chocolate {
             buildConfigField BOOLEAN, VARIABLE, FALSE
        }
    }

Loop the variants and check their names:

productFlavors {
    vanilla {}
    chocolate {}
}

applicationVariants.all { variant ->
    println("Iterating variant: " + variant.getName())
    if (variant.getName() == "chocolateDebug") {
        variant.buildConfigField "boolean", "VARIABLE", "true"
    } else {
        variant.buildConfigField "boolean", "VARIABLE", "false"
    }
}