We consider a two-parameter family of maps
T
α
,
β
:
[
0
,
1
]
→
[
0
,
1
]
with a neutral fixed point and a non-flat critical point. Building on a cone technique due to Baladi and Todd, we show that for a class of Lq
observables
ϕ
:
[
0
,
1
]
→
R
the bivariate map
(
α
,
β
)
↦
∫
0
1
ϕ
d
μ
α
,
β
, where
μ
α
,
β
denotes the invariant SRB measure, is differentiable in a certain parameter region, and establish a formula for its directional derivative.